an electron gun according to one embodiment of the present invention applied to a color picture tube will now be described in detail . referring to fig1 and 3 , there is shown an in - line electron gun assembly 1 . in these drawings , direction x is a direction parallel to the in - line direction of the electron gun assembly 1 , direction y is a direction perpendicular to both direction x and the tube axis , and direction z is a direction in which the tube axis extends and which is perpendicular to both directions x and y . fig2 is a sectional view of the electron gun 1 taken along a plane containing directions y and z , and fig3 is a sectional view of the electron gun 1 taken along a plane containing directions x and z . as shown in fig1 and 3 , the electron gun assembly 1 comprises a plurality of electrodes and two pairs of insulating support members 2a and 2b for supporting the electrodes . the electrodes include cathodes 9a , 9b and 9c arranged in line , first , second , third and fourth grids 11 , 12 , 13 and 14 , a convergence electrode 15 , and an auxiliary electrode 16 disposed between the third and fourth grids 13 and 14 and greater in size than the same . three heaters 6a , 6b and 6c for generating three electron beams 3a , 3b and 3c are arranged in the cathodes 9a , 9b and 9c , respectively . the three electron beams 3a , 3b and 3c generated by the heaters 6a , 6b and 6c in the cathodes 9a , 9b and 9c are passed through the electrodes 11 , 12 , 13 , 16 , 14 and 15 , respectively , and caused to hit against the red , green and blue phosphor layers ( not shown ) of a fluorescent screen as a target . the grids 11 to 14 and the convergence electrode 15 have apertures for passing through the electron beams as mentioned later and are unitized . the electron gun assembly 1 is formed of two fundamental sections : a crossover spot forming section , which includes a beam forming region , consisting of the cathodes 9 and the first and second grids 11 and 12 , and an accelerating and focusing lens section for focusing the electron beams on the screen . the crossover spot forming section may also be referred to as a four - pole section , which consists of the cathodes 9 and the first , second , and third grids 11 , 12 and 13 . the accelerating and focusing lens section is formally referred to as a main lens section , which consists of the third and fourth grids 13 and 14 . thus , the third grid 13 is used in common in the four - pole section and the main lens section . the construction of these electrodes will now be described in detail . the first and second grids 11 and 12 are planar in shape and arranged in close vicinity to each other . the third grid 13 , which is located close to the second grid 12 , is formed of two bathtub - shaped electrodes 23a and 23b which are joined together . the fourth grid 14 , which is located at a predetermined distance from the third grid 13 , is also formed of two bathtub - shaped electrodes 24a and 24b which are joined together . the convergence electrode 15 is formed of a single cup - shaped electrode 25a which is welded to the fourth grid 14 . three circular aperture are formed in each of the planar first and second grids 11 and 12 and the bottom face portions of both of the bathtub - shaped electrodes 23a , 23b , 24a and 24b of the third and fourth grids 13 and 14 and the cup - shaped electrode 25a of the convergence electrode 15 . each set of three apertures are aligned with their adjoining counterparts so as to be arranged along the paths of the individual electron beams . the apertures of the first and second grids 11 and 12 are relatively narrow , and the apertures 33a , 33b and 33c of the third grid 13 on the side facing the second grid 12 are greater than those of the first and second grids 11 and 12 . the apertures 43a , 43b and 43c of the third grid 13 on the side facing the fourth grid 14 , which are relatively wide , are equal in diameter to the apertures 34a , 34b and 34c of the fourth grid 14 on the side facing the third grid 13 . the apertures 35a , 35b and 35c of the convergence electrode 15 are narrower than the 43a , 43b and 43c of the third grid 13 and the apertures 34a , 34b and 34c of the fourth grid 14 . the auxiliary electrode 16 is formed of two bathtub - shaped electrodes 26a and 26b , and oval shaped apertures 36 and 46 are formed on the bottom faces of the bathtub - shaped electrodes 26a and 26b , respectively . the bathtub - shaped electrode 23b of the third grid 13 and the bathtub - shaped electrode 24a of the fourth grid 14 project into the apertures 36 and 46 , respectively . as shown in fig2 the bathtub - shaped electrodes 26a and 26b respectively have a pair of inside walls 98 and 99 respectively extending along the direction y from the peripheral walls of the bathtub - shaped electrodes 26a and 26b toward the beam plane . control elements are provided individually beside the apertures 44a and 44b of the convergence electrode 15 . the control elements are intended for satisfactory convergence of the three beams 3a , 3b and 3c on any portion of the surface of the screen . as shown in fig1 a bulb spacer 17 is attached to the outer periphery of the convergence electrode 15 . the bulb spacer 17 is supplied with a voltage as high as about 25 kv which is applied to an anode terminal ( not shown ). the electron gun assembly 1 constructed in this manner is sealed in a small cylindrical neck portion 18 which is formed of glass . a number of stem pins 19 are arranged at the left end portion ( fig1 ) of the neck portion 18 . the stem pins 19 support the electron gun 1 , and voltages for the grid electrodes 11 , 12 and 13 ( but not the convergence electrode 15 and the fourth grid 14 ) are externally applied through the stem pins 19 . in the electrode arrangement as aforesaid , the heaters 6a , 6b and 6c , the first , second and third grids 11 , 12 and 13 , and the one bathtub - shaped electrode 26a of the auxiliary electrode 16 are supported by the one parallel pair of insulating support means 2a . the other bathtub - shaped electrode 26b of the auxiliary electrode 16 and the fourth grid 14 are supported by the other pair of insulating support means 2b . the two bathtub - shaped electrodes 26a and 26b of the auxiliary electrode 16 are fixed at their flange portions 30a and 30b by welding . thus , the electron gun 1 is formed complete . in the electron gun assembly 1 with the construction described above , for example , the electrodes are supplied with voltages as follows . a cut - off voltage of about 150 v is applied to the cathodes 9 , and a modulation signal is added to the cut - off voltage . the first grid 11 is grounded , while voltages of about 700 v and 6 . 5 kv are applied to the second and third grids 12 and 13 , respectively . further , a high anode voltage of about 25 kv is applied to the fourth grid 14 , and a voltage intermediate between those applied to the third and fourth grids 13 and 14 is applied to the auxiliary electrode 16 . in the above described electron gun assembly 1 , the facing apertures 43a , 43b , 43c , 34a , 34b and 34c of the third and fourth grids 13 and 14 are made as wide as possible with the electron gun intervals kept narrow . further , electron lenses 100 , 101 and 102 shown in fig2 and 3 by broken dot lines are formed as long focal lenses which , under the influence of the potential of the auxiliary electrode 16 , produce an effect equivalent to that obtained when the distance between the third and fourth grids 13 and 14 is extended . a main lens section formed between the third and fourth grids 13 and 14 is protected against the influences of undesired electric fields in the neck 18 by the auxiliary electrode 16 . in the electron gun assembly 1 described above , however , the potential of the auxiliary electrode 16 may sometimes affect the electron lenses 100 , 101 and 102 unless the distance between the third and fourth grids 13 and 14 is shorter than the length of the auxiliary electrode 16 in the direction z . therefore , the spots of the three electron beams converged on the target through the electron lenses 100 , 101 and 102 may possibly be distorted in shape . in particular , the central electron lens 101 formed between the apertures 43b and 34b of the third and fourth grids 13 and 14 may be greatly influenced by the inside walls 98 and 99 of the auxiliary electrode 16 . it is difficult , moreover , to provide equivalent lens conditions for the central electron lens 101 and the two other electron lenses 100 and 102 formed between the apertures 43a and 43c of the third grid 13 and the apertures 34a and 34c of the fourth grid 14 . considering these circumstances , the inventor hereof made an additional improvement in the electron gun assembly 1 . namely , the electron gun assembly 1 has the following electrode arrangement in its main lens section . as indicated by the broken line in fig2 as well as in fig2 and 3 , platelike electrostatic field correcting electrodes 113 and 114 ( hereinafter referred to as correcting electrodes ) are welded to the opposite surfaces of the third and fourth grids 13 and 14 , respectively . the correcting electrodes 113 and 114 serve to correct the influence of the potential of the auxiliary electrode 16 on the main lens section . like the bathtub - shaped electrodes 23b and 24a of the third and fourth grids 13 and 14 , the correcting electrode 113 has three beam passage apertures 143a , 143b and 143c , and the correcting electrode 114 has apertures 134a , 134b and 134c . however , the correcting electrodes 113 and 114 greatly differ in shape from the bathtub - shaped electrodes 23b and 24a . namely , they are shaped so that the influence of the potential on the main lens section formed between the third and fourth grids 13 and 14 is controlled . fig4 is a sectional view taken along line iv -- iv of fig1 showing the one bathtub - shaped electrode 26a of the auxiliary electrode 16 and the correcting electrode 113 . as shown in fig2 to 4 , the correcting electrode 113 is disposed within the bathtub - shaped electrode 26a . in fig4 an outline of the bathtub - shaped electrode 23b is indicated by a broken line . the correcting electrode 113 is attached to that portion of the bottom surface of the bathtub - shaped electrode 23b which faces the bathtub - shaped electrode 24a . as shown in fig4 the x - direction dimension of the correcting electrode 113 is substantially equal to the bathtub - shaped electrode 23b , but the y - direction dimension of the correcting electrode 113 is greater than that of the bathtub - shaped electrode 23b . the correcting electrode 113 has a projection 144 projecting over a substantial distance in the direction y from its central portion . the correcting electrode 114 mounted on the fourth grid 14 is similar to the correcting electrode 13 shown in fig4 in shape . the correcting electrode 113 mounted on the third grid 13 is applied with the same voltage as the one applied to the third grid 13 , and the correcting electrode 114 on the fourth grid 14 with the same voltage as the one applied to the fourth grid 14 . according to the present invention , the attachment of the correcting electrodes 113 and 114 to the third and fourth grids 13 and 14 causes the central electron lens 101 to be hardly influenced by the electrostatic field of the auxiliary electrode 16 , especially that of the walls 98 and 99 of the auxiliary electrode 16 . thus , the electron beam 3b passed through the central electron lens 101 forms a circular beam spot on the target after undergoing a lens action . on the other hand , the two other electron lenses 100 and 102 are moderately influenced by the potential of the auxiliary electrode 16 in electrostatic fields . the electron beams 3a and 3c passed through the electron lenses 100 and 102 are converged so as to be bent toward the central electron beam 3b . then , the electron beams 3a and 3c form a substantially circular beam spot on the target . the three electron beams 3a , 3b and 3c converge on a common spot on the target . fig5 a , 5b and 5c respectively show the shape of the spots of the electron beams 3a , 3b and 3c . as shown in fig5 a , 5b and 5c , each of the three beam spots 103a , 103b and 103c includes a substantially circular bright point ( hatched portion ) and a substantially circular halo portion ( outline by full line ) without any substantial distortion . for comparison , the bright point and halo portion of each beam spot obtained without the use of the correcting electrodes 113 and 114 in the electron gun 1 are indicated by a broken line and a dashed line , respectively . in this embodiment , if the length of the auxiliary electrode 16 in the direction z is not sufficiently longer than the distance between the third and fourth grids 13 and 14 , that is , if the inside walls 98 and 99 of the auxiliary electrode 16 are located close to the facing bottom ends of the third and fourth grids 13 and 14 , then the influence of the potential of the auxiliary electrode 16 on the electron lenses 100 , 101 and 102 is controlled for proper correction by the correcting electrodes 113 and 114 . as a result , the distortions of the electron beams 3a , 3b and 3c converged on the target are removed . in the embodiment described above , the correcting electrodes 113 and 114 are mounted as platelike electrodes on the opposite surfaces of the bathtub - shaped electrodes 23b and 24a of the third and fourth grids 13 and 14 , respectively . alternatively , according to the present invention , correcting electrodes 160 may be attached respectively to the outer surfaces of the longitudinal side walls of the bathtub - shaped electrode 23b , as shown in fig6 . each of the correcting electrodes 160 has an l - shaped cross section in the direction z , including a projection 162 which extends along the opposite surface of the bathtub - shaped electrode 23b of the third grid 13 in the vicinity of the central aperture 43b thereof . in the above described embodiment , the correcting electrodes 113 and 114 are each in the form of a flat plate . alternatively , however , correcting electrodes 170 and 172 , whose cross section in the direction , y is curved along the direction y , may be provided . namely , as shown in fig7 both ends of the one correcting electrode 170 in the direction y extend from its junction with the third grid 13 toward the other correcting electrode 172 , and those of the other correcting electrode 172 extend from its junction with the fourth grid 14 toward the one correcting electrode 170 . the shape of the correcting electrodes 170 and 172 is especially effective if the distance in the direction y between the walls 98 or 99 of the auxiliary electrode 16 and the opposite end of the outer peripheral wall of the bathtub - shaped electrodes 23b or 24a is short . in the embodiments shown in fig1 to 4 and 7 , the two correcting electrodes 113 and 114 ( 170 and 172 ) are mounted on the longitudinal side walls of the third and fourth grids 13 and 14 , respectively . alternatively , however , a single correcting electrode may be provided on the third or fourth grid 13 or 14 , depending on the relative positions of the auxiliary electrode 16 and the third and fourth grids 13 and 14 . according to the present invention , moreover , a pair of correcting electrodes 180 , as shown in fig8 may be attached individually to the outer surfaces of the side walls of the bathtub - shaped electrode 23b in the same manner as in the case of the correcting electrodes 160 shown in fig6 . each of the correcting electrodes 180 includes a projection 182 which extends along the direction z in the vicinity of the central aperture 43b . the projections 182 of the correcting electrodes 180 make it possible to correct the influence of the electrostatic field of the auxiliary electrode 16 , as in the cases of the foregoing embodiments . according to the present invention , moreover , each of the correcting electrodes may be formed integrally with the third or fourth electrode . in all the embodiments described above , the arrangement of the electron lenses is based on a bipotential lens which consists of the third and fourth electrodes 13 and 14 . the present invention is not , however , limited to such an arrangement , and may also be applied to electron lenses of various other types , such as unipotential , quadra - potential , periodic - potential , and tri - potential electron lenses . further , a resistor may be provided in the vicinity of the electron gun in the neck portion of the cathode ray so that the high voltage of the fourth grid 14 is divided by the resistor , whereby the third grid 13 and the auxiliary electrode 16 are supplied with voltages . in the electron gun assembly of the embodiments described above , furthermore , three electron guns are arranged transversely in line . alternatively , however , the three electron guns may be arranged in a delta , or more electron guns may be arranged in some configuration . the present invention may also be applied to a cathode - ray tube including a single electron gun .