there is shown in fig1 an ophthalmodynamometer comprising support assembly 12 , transducer assembly 14 , oscillator 16 , and demodulation circuitry 21 . a subject bites on bite board 10 connected by support assembly 12 to transducer assembly 14 . the transducer assembly is positioned so that the subject stares straight ahead into it but also so that the corneal surface does not touch it . while apparatus for use with one eye is described with reference to fig1 additional transducers and circuitry may be provided so that both eyes may be concurrently examined , as will be described with reference to fig9 . oscillator 16 produces a continuous sinusoidal voltage with frequency f c equal to 1mhz . the voltage is of the form sin ( w c t ) where w c equals 2πf c . this voltage is coupled into focused transmitter 18 of transducer assembly 14 , which produces continuous ultrasonic wave energy having a frequency of 1mhz that is propagated to the surface of the eye . the transmitted energy is substantially collimated and about 2 to 3 mm . in diameter , starting from about 21 . 75 mm . from transmitter 18 and continuing for a depth of field of about 15 mm . the distance from the transducer assembly to the eye is preferably chosen from 21 . 75 to 36 mm . so that the corneal surface will be somewhere within the depth of field . for transducer assembly 14 , optimum return signals are obtained in this distance range . the energy is reflected from the moving corneal surface , and received by focused receiver 20 of assembly 14 . because the corneal surface moves toward , then away from , assembly 14 due to its distention and contraction by the ocular pulse , continuous wave ultrasonic energy hitting this moving surface will be reflected back at a different frequency because of the doppler effect . specifically , the shift or change in reflected carrier frequency ( δf c ) of an incident wave front striking a moving surface is given by : ## equ1 ## where δf c = change in interrogation frequency f c , v ( t ) = velocity of the object being interrogated , a function of time t , the reflected beam is thus frequency modulated , because the frequency shift , δf c , carries information about corneal surface velocity . receiver 20 converts the ultrasonic return energy back to an electrical signal of the form a sin [( w c + δ w c ) t ], where δw c , the angular frequency shift , equals 2πδf c and a is the varying peak amplitude of the return signal . this signal is then coupled into demodulation circuitry 21 , which comprises preamplifier - filter 22 , phase locked loop 24 , high - pass filter 26 , low - pass filter 28 , integrator 30 and recorder 32 . the signal is first coupled into preamplifier - filter 22 , which has a band - pass filter stage and an automatic gain - controlled amplifier stage . the band - pass filter stage has low and high frequency breakpoints bracketing the modulated frequency ( w c + δw c ) in order to eliminate noise . the amplifier stage multiplies the input signal by a varying gain ( δg ) which is inversely proportional to the varying amplitude a to provide a signal having a substantially constant peak amplitude . the output of preamplifier - filter 22 , represented by δga sin [( w c + δw c ) t ], is then coupled into phase locked loop 24 . the phase locked loop demodulates the frequency modulated signal by extracting from the signal an output which is proportional to the frequency shift , δw c . the loop accomplishes this through the following feedback procedure . the loop initially generates its own reference signal , w l , which it compares with an incoming signal to determine both the frequency and phase difference between the signals . when there is a frequency difference , the loop will generate an error voltage δv o that is proportional to the frequency difference . δv o is part of the d . c . output of the loop , v o . v o is fed back to vary the loop reference frequency w l until it equals the incoming signal frequency and to change the phase of the reference signal until it is exactly 90 ° out of phase with the incoming signal . the loop reference signal is then &# 34 ; locked &# 34 ; into this phase relationship with the incoming signal , and thus it will follow any subsequent change of phase in the incoming signal . the whole feedback procedure is repeated for subsequent frequency changes in the incoming signal . it follows that when the loop reference frequency w 1 and the frequency w c of the wave transmitted to the eye are initially equal , which is accomplished by adjusting the frequency of oscillator 16 so that w c equals w l , the loop output will be proportional to the doppler shift , δw c . that is , δv o = the change in the phase locked loop demodulated output voltage , δw c = the change in the input carrier frequency , and k o = the conversion ( d . c . to a . c .) gain of the phase locked loop . combining equations ( 1 ) and ( 2 ) the changing d . c . voltage is given by , ## equ2 ## θ , the angle of incidence , varies so little from 0 ° when the eye is staring at the transducer that it may be considered a constant . thus the demodulated output of the phase locked loop is proportional to the velocity of the surface being interrogated . that is , where k = 4πcos θ / ck o is a contant . the signal δv o is thus a direct representation of the velocity of the corneal surface as a function of time . the signal coming from the phase locked loop may contain unwanted artifacts arising in the following two ways : first , slow , varying motion in either or both the ambient air and the corneal surface ( not caused by the ocular pulse ) shows up as a very low frequency signal ( normally about 0 . 5 hz ) on which the ocular pulse signal ( about 5 to 10hz ) is superimposed ; and second , the high frequency ( here 1mhz ), low amplitude loop reference signal w l used to do frequency and phase comparison in the loop is superimposed on the ocular pulse output signal . high - pass filter 26 removes this slow varying , low frequency component coming from loop 24 , and low - pass filter 28 removes this high frequency carrier component . thus the combined effect of filters 26 and 28 , which are actually amplifiers with unity gain above and below respective low and high frequency breakpoints , is to provide a band - pass filter allowing only the ocular pulse , corneal velocity signal to get through . to obtain a sharply defined band - pass around the ocular pulse signal so as to eliminate the maximum amount of noise , filters 26 and 28 are chosen to produce a butterworth - thomson response curve . the output of filter 28 is coupled into recorder 32 to give a readout of the velocity of the corneal surface as a function of time . the output of filter 28 is also coupled into integrator 30 , which integrates the velocity - time output to provide a displacement - time signal , which is then coupled into recorder 32 to give a readout of the position ( displacement ) of the corneal surface as a function of time . integrator 30 acts as a simple constant gain amplifier for frequencies lower than the ocular pulse to provide d . c . stability . the additional arrows into recorder 32 indicate reference wave inputs such as the electrocardiogram . there is shown in fig2 and 3 a more detailed view of assembly 14 . the transducer housing comprises a hollow plastic tube 34 press - fitted within metal sleeve 36 having a thickness of 0 . 2 mm . and an inner diameter of 20 mm ., the housing having a total length of 35 mm . tightly fitted into the end of sleeve 36 is cork ring 38 having a hole diameter of 6 . 0 mm . and an axial length of 10 mm . an annular section 15 . 6 mm . in diameter and 6 mm . inward from the end of sleeve 36 has been removed from ring 38 to receive a metal tube 40 of 0 . 2 mm . thickness and 7 mm . long . about a 1 mm . deep circular groove has been cut into ring 38 to seat tube 40 . press fitted within ring 38 is focused transducer receiver 20 , a piezoelectric ring made of barium titanate ( ba ti o 3 ) and having in its outer face a spherical concave shape with a radius of curvature of 21 . 75 mm . the hole diameter of the receiver is 6 . 0 mm . spaced from cork ring 38 by about 5 mm . is cork ring 42 having a hole diameter of 5 . 4 mm . and being about 5 mm . thick . fitted within ring 42 is metal tube 44 , about 24 mm . long and extending through the hole in cork ring 38 . fitted into the end of tube 44 so that its back face is aligned with that of receiver 20 is focused transducer transmitter 18 , a piezoelectric cylinder of barium titanate . the front face is spherical and concave with the same radius of curvature as has receiver 20 . an epoxy resin layer 46 of a thickness of 0 . 2 mm . coats the concave faces of transmitter 18 and receiver 20 . because its index of refraction is intermediate to that of air and barium titanate , it serves to lower the air - crystal impedance mismatch so that ultrasound may be coupled between the transducer and air with minimum losses due to reflection at the interface . the coating may be any thickness which is an integral multiple of 1 / 4 of the ultrasonic wavelength , and will thus act as an interference filter . isolating metal ring 44 and transmitter 18 from receiver 20 is annular air gap 48 , 0 . 3 mm . wide . deposited on the back face of transmitter 18 and receiver 20 is a thin conductive metal coating , to which are respectively soldered transmitter lead 50 and receiver lead 52 . transmitter ground lead 54 and receiver ground lead 56 , which are isolated from each other by ring 42 and by plastic tube 34 , are respectively connected to tube 44 and tube 40 . coaxial cables 58 , 60 with their metal connectors 58a , 60a fitted respectively through apertures in tube 34 and sleeve 36 , respectively couple the transmitter leads to oscillator 16 and the receiver leads to preamplifier - filter 22 . in operation , a sinusoidal signal from oscillator 16 enters transmitter 18 through lead 50 , causing the resonant crystal to vibrate and thereby emit continuous wave ultrasonic energy having a frequency of 1mhz . cork rings 38 and 42 and air gap 48 provide primarily acoustical but also electrical and mechanical isolation between transmitter 18 and receiver 20 so that noise due to cross talk of all types is kept down to an acceptable amount of about 1 microvolt . the emitted waves , as previously noted , are collimated over a depth of field into a 2 to 3 mm . diameter beam . if the eye is placed within the depth of field , the beam strikes the cornea over a small and very gently curved area so that the phase delays and resultant interference between reflected waves are small and hardly affect the return signal . also , the angle of incidence varies at most by minutes of arc from 0 ° so that substantially all of what is reflected back out of this on - axis signal will be collected by focused receiver 20 . the corneal tissue -- air interface creates a substantial impedance mismatch for ultrasound so that very little energy is transmitted intraocularly , most being reflected from the corneal surface . although the power level used is safely at least two orders of magnitude less than 2 . 5 watts / cm 2 , the damage threshold to the cornea , there is sufficient return signal to be easily detected . likewise , the attenuation of the ultrasonic signal in the air , which is severe at these high frequencies even over the small distances here involved , has little effect on the quality of the return signal , which , because it is frequency - modulated , carries information that is independent of the amplitude . the position of transducer assembly 14 is adjustable in support assembly 12 ( fig4 ), comprising clamp 62 , support poles 64 , bite board 10 , and posts 70 . clamp 62 has a pair of curved clamping members 62a which by means of screw 66 grip the transducer housing . clamp 62 is freely moveable , and , in cooperation with adjustable clamp supports 68 , allows transducer assembly 14 to be raised , lowered , moved laterally and toward or away from the eye . support poles 64 are connected to wooden bite board 10 , which in turn is supported by posts 70 . by having the subject bite bite board 10 , the head is held stationary while the transducer is positioned . the optimum distance between eye and transducer is obtained by displaying the signal reflected from the eye on an oscilloscope before the signal is processed by elements 22 to 30 . with the subject staring straight ahead , it requires a few seconds to adjust the position of transducer assembly 14 until the return signal is maximized . the return signal is then processed by the apparatus of fig1 with a standard grass polygraph serving as recorder 32 . a typical velocity - time readout is shown in fig5 and a typical distance - time readout shown in fig6 . in fig5 ( a ) shows a human subject &# 39 ; s electrocardiogram ( ecg ), taken in the einthoven triangle configuration with the upper leads on the lateral chest wall , ( b ) is a one - second time marker , ( c ) is the corneal surface velocity - time plot , and ( d ) is a myographic trace of extraocular muscle activity . the almost sinusoidal pulsations in ( c ) represent the velocity of the corneal surface as it is distended and then undergoes a damped oscillation back to equilibrum . the profile shown is that of free - running corneal motion , since no weight or other pressure biases the natural eye movement responsive to the ocular pulse . it is well known that the systolic peak in the aorta occurs near the beginning of the t - wave , designated t in ( a ). it can be seen that the ocular pulsations in ( c ) are contiguous with this region of the ecg , as would be expected . the low level myographic trace ( d ) due to the signal picked up by electrodes connected around the orbit of the eye shows that the signal in ( c ) is not caused by involuntary muscular eye movement , which produces a phase shift on the order of 1 / 1000 of the ultrasonic wavelength when staring , whereas the ocular pulse is on the order of 1 / 4 wavelength and produces a signal about 250 times stronger . even when the eye is tracking the motion of a sine wave on an oscilloscope , the noise caused by this motion is still well below the ocular pulse signal level , since the phase shift caused by tracking is about 1 / 500 of a wavelength . in fig6 ( w ) is a human subject &# 39 ; s ecg , ( x ) is the one - second time marker , ( y ) is the corneal surface displacement ( from the transducer )- time plot , and ( z ) is a graph of the carotid pulse made by a conventional pressure transducer touching the neck . the biphasic appearance of each pulse in ( y ), with the dicrotic notch between each pair of peaks , is characteristic of the ocular pulse . as with the velocity profile , the displacement profile is correlated to the ecg and its t - wave , designated t in ( w ), but it is also correlated to the carotid pulse displayed in ( z ). fig7 shows a circuit corresponding to the block diagram of fig1 . oscillator 16 is a general radio model 1310 - a oscillator . preamplifier - filter 22 is described in more detail in fig8 . phase locked loop 24 is a signetics ne 562 b phase locked loop ( integrated circuit ) with its terminals connected as shown . high - pass filter 26 and low - pass filter 28 each employ national semiconductor operational amplifier lm 210 h with the terminals connected as shown to perform the high - pass function in 26 and the low - pass function in 28 . integrator 30 employs national semiconductor operational amplifier lm 201 ah with the terminals connected as shown to perform the integration function . resistor r f provides negative feedback and thus d . c . amplifier amplifier stability for signals of lower frequency than the ocular pulse . circuitry for preamplifier - filter 22 as shown in fig8 comprises two stages , a band - pass filter 22a and an automatic gain - controlled amplifier 22b . band - pass filter 22a uses national semiconductor operational amplifier lm381n with the terminals connected as shown , and amplifier 22b uses national semiconductor operational amplifiers lm370n and lm733cn and a 2n3904 transistor all connected as shown . though not preferred , a tektronix model 561b oscilloscope with preamplifier model 3a9 could be used as preamplifier - filter 22 . the arrangement of the apparatus for simultaneous measurement of the ocular pulses of both eyes is shown in fig9 . a pair of matched transducer transmitters and receivers powered by a common oscillator 16 and mounted on support assembly 12 , adapted to hold two transducer assemblies , with two sets of receiver circuitry identical to that described for one eye is utilized , with the addition of function module 31 connected between low - pass filters 28 and recorder 32 and also between integrators 30 and recorder 32 . function module 31 is a subtracter whose output is the difference between the right and left waves , although a variety of analog or digital devices could be used here depending on how one wished to process the two waves . as shown , the right and left outputs also bypass the function module to be fed directly into recorder 32 . also shown are inputs 1 , 2 , and 3 to be the recorder , for the ecg , the carotid pulse , and other desired reference waveforms . regarding modifications in application and structure , the ultrasonic signal can also be beamed against the sclera of the eye , although because of the nature of this tissue more damping occurs so that the return signal is not as strong as that reflected from the cornea . as to distance between the eye and transducer , the farther the signal must travel , the more energy must be put into the signal so that the danger of transmitting too much energy into the eye increases at long distances . preferably the depth of field should range from 15 to 50 mm ., the specific value being determined by the f / no . of the transmitter and the ultrasonic frequency selected . for a frequency range of 100khz to 10mhz , the f / no . can range from 1 to 25 . as to the transducers , any commercially available ultrasonic materials , especially piezoelectric materials , which can be made to exhibit resonances in the preferred frequency range are suitable . the following are included : pure piezoelectric materials such as quartz or rochelle salt ; piezoelectric ceramics of perovskite , distorted perovskite , or non - perovskite structure ( e . g . ba ti o 3 , pb ( ti , zr ) o 3 , na n b o 3 , and na n b o 3 with k , pb , or cd substituted for na ); magnetic ceramics which can generate ultrasound at the desired frequencies ; and other materials which can be used to generate ultrasound such as the ferromagnetic elements , ferrites , and ferroelectric alloys . as to the air - transducer impedance matching coating , any material or materials , used singly or in discrete layers ( the latter having progressively increasing indices of refraction going from the air to the transducer surface ), are suitable . other conventional acoustical damping materials besides cork can be used such as aerated polymer plastics . other configurations besides the two - piece spherical transducer can be used . the receiver can be a segmented array for multi - channel reception . the curvature of the transducer parts can be elliptical , parabolic , or any shape that would maintain acoustical beam integrity across some distance , and indeed the transducer can be flat or otherwise unfocused , although in that case accurate alignment of eye and transducer becomes a more serious consideration . in the transducer assembly described the transmitter could be used as the receiver and the receiver as the transmitter . as to the method of modulation , frequency modulation ( f . m .) offers the best results , but amplitude modulation ( a . m .) can also be used . a change in the cornea - transducer distance caused by the ocular pulse will vary the amplitude of the return signal , the weakest ( lowest amplitude ) signals travelling the farthest and being most attenuated in the air . furthermore , the total amount of reflected signal ( backscatter ) will decrease with increasing distance to the reflecting surface . therefore a continuous measurement of the amount of energy in the return signal can be correlated to changing cornea - transducer distance . circuitry similar to that described but arranged to perform amplitude demodulation can be used . because cornea - transducer distance is critical to a . m ., unwanted eye motion presents a serious problem to effective use of this method , and must be carefully controlled . the testing time is a function of the pathology to be diagnosed . some , such as glaucoma or carotid stenosis , will require application of the signal for only about 10 seconds . others may require somewhat longer time , but in most cases this time will not exceed 30 seconds . if necessary , repeated or continuous runs can be taken for as long as desired . other support assemblies for positioning the transducer in spaced relation to the eye besides support assembly 12 can be used such as a headpiece with a built - in pair of transducers . the invention need not be restricted to measuring the ocular pulse . biological movements throughout the body can be measured such as the radial pulse and vascular blood flow . such readings may , however , not be as free from noise as the ocular pulse , one reason being intervening tissue between the transducer and the source of motion . the invention can alternatively be used in the conventional touch mode by placing the transducer on the neck or wrist , for example , to measure the carotid or radial pulse . other embodiments within the invention will be apparent to those skilled in the art .