690 SUMMARY OF CURRENT RESEARCHES RELATING TO 



it is not necessary to cover up half of each of the eye-ijiece tubes, thus 

 losing half the total amount of liglit. It is sufficient if one only (the 

 lateral one) is half obscured, leaving the other free. As the normal 

 division of light between the two tubes is two-thirds (in the axial) 

 and one-third (in the lateral), the total loss of light is reduced to 

 one-sixth. 



The field of view of the axial eye-piece in this arrangement in 

 any case necessarily appears brighter than that of the lateral one seen 

 with the same eye ; and in regard to this. Professor Abbe remarks * that 

 the difference between the brightness of the two fields in binocular 

 observation " is not only no defect, but on the contrary a decided 

 advantage. For experience has long proved that to obtain a good 

 stereoscopic effect it is only necessary that one image should be as 

 perfect and clear as possible, whilst the other may, without appreciable 

 disadvantage, be of sensibly less perfection.^ It might therefore be 

 anticipated that this would apply (as in fact it does) in the same way 

 to difference of luminosity. Moreover, an additional fact must be 

 taken into account — that the two eyes, especially of microscopists, 

 always show unequal sensibility to light as the result of constant un- 

 equal use. The less used eye, whose acuteness of vision is always less 

 than that of the one more frequently exercised, shows a greater sensi- 

 bility to light, and the difference is so considerable that the less lumi- 

 nous image of the lateral eye-piece, when viewed with the less 

 exercised (generally the left) eye, seems even brighter than the other 

 when viewed with the exercised eye. The unequal division of the 

 light is therefore a welcome element, as it serves to equalize this 

 physiological difference. The observer has only to take care that the 

 less used eye is applied to the lateral eye-piece." 



Illumination for Binocular Microscopes with High Powers.! 



— Eeferring to the plan explained in the preceding note for half 

 covering up only one of the eye-pieces, Professor Abbe says : — " On 

 the other hand, there are cases — especially when high powers are 

 used for binocular observation — where the simultaneous covering up 

 of both eye-pieces may be of good service, whilst at the same time the 

 loss of light may be fully compensated by the method of illumina- 

 tion. All stereoscopic vision with the Microscope, as far as it is any- 

 thing more than mere seeing with tivo eyes, depends exclusively upon 

 the unequal inclination of the pencils which form the two images, to 

 the plane of the preparation or the axis of the Microscope. By uni- 

 form halving of the pencils, whether by prisms above the objective or 

 by diaphragms over the eye-pieces, the difference in the directions of 

 the illumination in regard to the preparation reaches approximately 

 the half of the angle of aperture of the objective, provided that its 

 whole aperture is filled with rays. By the one-sided halving we have 

 been considering, the direct image is produced by a pencil the axis of 

 which is perpendicular to the plane of the preparation, and the de- 

 flected image by one whose axis is inclined about a fourth of the angle 



* Zeitachr. f. Mikr., ii. (1880) p. 207. 



t Cf. Carpenter, ' The Microscope, &c.,' 6th ed. (1881) p. 36. 



i Zeitschr. f. Mikr.. ii. (1880) p. 207. 



