prior to the description of the preferred embodiments of the photographic stroboscope in accordance with the present invention , a typical prior art stroboscope provided with a flash delaying means will be briefly described with reference to fig1 . in response to the flash signal from a camera ( not shown ) applied to a terminal 12 , an electronic or mechanical switch 9 is enabled so that the charging of a capacitor 11 is started and the voltage across the capacitor 11 increases depending on a time constant which is determined by the values of a resistor 10 and the capacitor 11 . when the voltage across the capacitor 11 increases and reaches an inherent conduction voltage of a trigger element 8 such as a diac , zener diode , neon bulb or the like , the discharging current from the capacitor 11 flows into the gate of a thyristor 6 to enable the same so that the energy which has been stored on a trigger capacitor 4 through a resistor 3 is liberated and consequently a flash lamp 2 flashes by a high - voltage pulse induced across a secondary of a trigger transformer 5 . a time delay t elapsing from the time the signal from the camera is applied to the terminal 12 to the time the flash lamp 2 flashes is dependent upon a time required for the capacitor 11 to charge the voltage thereacross equal to the conduction voltage of the trigger element 8 . that is , the conduction voltage e of the trigger element 8 is given by ## equ1 ## where v = a voltage charged across a main capacitor 1 , since c , r and e are constants , the time delay t is solely dependent on the voltage charged across the main capacitor 1 . therefore , in order to render the time delay constant irrespective of the variation in the voltage charged across the main capacitor 1 , a reference voltage source 13 must be provided which is free from the influence of the voltage charged across the main capacitor 1 . as a result , the prior art stroboscope is much complicated in circuit design and accordingly is expensive . in addition , during the assembly a floating capacitance is invelved between the terminal and the gate of the thyristor 6 so that in response to the impression of the command signal on the terminal 12 the thyristor 6 is caused to be enabled before a predetermined time delay has been elasped due to this floating capacitance and consequently the &# 34 ; mis - flashing &# 34 ; results . the first embodiment of a photographic stroboscope in accordance with the present invention includes a flash delay means generally indicated by the reference numeral 23 and comprised of a terminal 14 to which is applied a command signal from a camera ( not shown ), a differentiating circuit consisting of a capacitor 16 and a resistor 17 , a first switching circuit consisting of a resistor 15 and a semiconductor switching element 18 such as a transistor , a second switching circuit consisting of a not gate 19 and a nand gate 20 , a capacitor 21 and a variable resistor 22 . the differentiating circuit and the first switching circuit comprise a first switching means . the input to not gate 19 is maintained at a low level until the command signal is applied to the terminal 14 so that the output of not gate 19 is maintained at a high level . the transistor 18 is disabled so that its collector output is at a high level and the output from nand gate 20 is at a low level . as a result , the capacitor 21 is not charged . when the command signal is applied to the terminal 14 , it is applied , after being differentiated by the differentiating circuit consisting of the capacitor 16 and the resistor 17 , to the base of the transistor 18 to enable it so that the collector output drops to a low level and the output of nand gate 20 rises to a high level . as a consequence , the charging current flows through nand gate 20 and the variable resistor 22 to the capacitor 21 so that the latter is charged with the polarities shown . the voltage across the variable resistor 22 causes the gate of the thyristor 6 to reverse biase with respect to the cathode of the thyristor 6 so that the latter is not enabled . the command signal is applied to the terminal 14 only for a very short time so that the transistor which has been enabled as described above is soon disabled to interrupt the current flowing through the variable resistor 22 . then the output of not gate 19 rises again to a high level so that the output of nand gate 20 drops again to a low level so that the energy stored on the capacitor 21 is discharged through the variable resistor 22 . in this case , the voltage across the variable resistor 22 has the polarities shown in fig2 so that the thyristor 6 is forward biased and is enabled and consequently the flash lamp 2 flashes . the value of the variable resistor 22 determines both the charging and discharging time of the capacitor 21 and therefore the time delay t ; that is , the time interval elapsing from the time the command signal is applied to the terminal 14 to the time the flash lamp 2 flashes . that is , the delay time t is given by v &# 39 ; = a threshold voltage applied to the input of the not gate 19 , and v = a source voltage applied to the not gate 19 . in general , the threshold voltage v &# 39 ; is approximately 1 / 2 of the source voltage v . therefore , substituting v &# 39 ; = 1 / 2v into eq . ( 3 ), we have ## equ2 ## thus the time delay t is solely dependent on the capacitance of the capacitor 21 and the value of the variable resistor 22 . as described above , in the photographic stroboscope in accordance with the present invention , the time delay t ; that is , the time interval elapsing from the time the command signal is applied to the terminal 14 to the time the flash lamp flashes is solely determined by the time constant of the capacitor 21 and the variable resistor 22 so that the time delay t may be maintained constant in a stable manner without the provision of a reference voltage circuit . furthermore , the gate of the thyristor 6 is reverse biased from the time the command signal is applied to the time the thyristor , which is a switching element in a trigger circuit , so that an erratic operation ; that is , the conduction of the thyristor immediately after the application of the command signal to the terminal 14 may be avoided . in the second embodiment of a photographic stroboscope in accordance with the present invention , in response to the application of the command signal to the terminal 12 , the mechanical or electronic switching circuit 9 is closed so that a trigger capacitor 25 is charged through a constant current circuit 24 . when the voltage across the trigger capacitor 25 reaches the conduction voltage of the trigger element 8 , the latter is enabled so that the current discharged from the trigger capacitor 25 flows into the gate of the thyristor 6 to enable it to conduct . as a result , the energy stored on the trigger capacitor 25 is discharged through a primary of a trigger or step - up transformer 5 so that a high - voltage pulse induced across the secondary of the trigger transformer 5 causes the flash lamp 2 to flash . the time delay t as defined in the second embodiment as the time interval elapsing from the time the command signal is applied to the terminal 12 to the time the flash lamps 2 flashes is dependent upon the time required for the voltage charged across the trigger capacitor 25 to reach the conduction voltage of the trigger element 8 , and is given by all of e , c and i are constants so that the time delay t is not affected by the voltage v charged across the main capacitor 1 and consequently a constant voltage source may be eliminated from the delay circuit . the delay time t may be varied by varing the output current i from the constant current circuit 24 so that the photographic stroboscopes in accordance with the present invention may be advantageously applied to a variety of pocket type cameras in which a time interval from the time a flash command signal is applied to the time the shutter is wide opened is different from each other .