SUGGESTED IMPROVEMENT IX BINOCULAR MICROSCOPES. 385- 



that no separate part of an objective can "look" along any- 

 other direction than one parallel to the optic axis of the whole 

 lens — a very different matter from " looking at" the object from 

 the actual direction of the part of the objective utilised, in which 

 case the object squares we were considering just now would be 

 foreshortened, and assume different shapes according to the point 

 from which they were regarded. Helmholtx appears to have 

 recognised, almost half a century ago, the peculiar manner in 

 which the different parts of an objective " look at " and " see " 

 the object, for in his " Physiological Optics " * the action of 

 Nachet's binocular microscope is explained as due to the causes 

 stated, in a few crisp and short sentences. But no better proof 

 can be given that his explanations were not understood till a 

 much later date than that Naegeli and Schwendener, in their 

 well-known work on the microscope, dismiss Helmholtz's remarks 

 in a short footnote as being incorrect. f 



The first to explain the whole matter at length was Abbe, who, 

 in a series of papers in 1881 and 1882, notably in his paper 

 " On the Mode of Vision with Objectives of Wide Aperture," j 

 clearly showed how the lateral shifting of the images of different 

 planes of the object by different parts of the objective constitutes 

 a particular form of parallactic displacements. Great controversies 

 were going on in those days on the subject of " all-round ' ; 

 vision, wide-angled lenses being supposed to be able to look 

 round a small object more than narrow-angled lenses ; and Abbe's 

 paper, by showing just exactly in what way the different parts 

 of the objective do delineate the parts of an object, very 

 materially elucidated this question. But as a perusal of 

 Carpenter's Handbook shows, opinions on these matters continued 

 to be very divided, even after the publication of Abbe's papers. 



The following point, which I have not found mentioned 

 anywhere, seems to be one worth noticing, because it is one 

 giving results which might easily be wrongly interpreted. 

 Imagine a small cube placed on the object-stage of the micro- 

 scope, so that the optic axis of the objective passes through the 



* Handbuch der PhysiologiscTien Oj/tih. 1867, p. 682. 



+ The Microscope, Xaegeli & Schwendener. English translation, 

 2nd edition, 1892, p. 226. (The German edition of this work is, 

 however, of an earlier date — viz. 1877.) 



J Journal Royal Microscopical Society, 1884, pp. 20—26. 



