SUGGESTED IMPROVEMENT IN BINOCULAR MICROSCOPES. 377 



On the other hand, difference of inclination, with respect to 

 the optic axis of the instrument, makes a very appreciable differ- 

 ence in the perspective — a difference the more accentuated because, 

 as we shall see a little further on, the apparent depth dimension 

 of an object is exaggerated, as compared with the magnification 

 of horizontal surfaces. This ' ; perspective of inclination " would, 

 in fact, seem to be the chief aid to our sense of solidity, when 

 we get a number of objects of similar shapes in the field of 

 view at the same time — as, for example, when we have a slide 

 of Polycystina. 



Comparison with objects of known form and position will, of 

 course, aid us in vision with the microscope just as with the 

 unaided eye ; and though in general, owing to the restricted field 

 of view, we may not have the known comparison objects at hand 

 so frequently, yet the unconscious comparison with objects we are 

 familiar with in ordinary vision will materially aid us in our 

 conception of the form — in fact, may assist us to overcome any 

 optical distortion which may be present. 



Nos. 4 and 6 in our list of causes of impression of solidity have 

 to be practically dispensed with. The pupil of the eye, when 

 looking through a microscope, is placed near the position of the 

 Eamsden circle of the microscope — the little bright ring of light, 

 seen just above the eye-piece — and even with the lowest powers 

 .and low eye-pieces the size of this is considerably less than 

 that of the pupil of the eye. Therefore there is little scope for 

 lateral movements of the eye, and for any relative displacements 

 of things seen, from this cause ; although slight displacements 

 may possibly occur owing to a special form of ocular parallax 

 recently elucidated by Professor W. F. Barrett.* Different 

 colours at different planes to help in the conception of 

 solidity are rarely met with in the microscope in ordinary 

 practice. 



Shadow effects are mostly met with under such very different 

 conditions to those in vogue in ordinary vision — because in general 

 we either use transmitted light with the microscope, or throw the 

 light on the objects from all sides as in dark ground illumination 

 — that little reliable judgment can be formed from these under 

 ordinary circumstances, although by using oblique illumination 



* "On Eutoptic Vision,'' Scientific Proceeding* lioijal Dublin Society, 

 1906, vol. xi., No. 8, p. 76. 



