SUGGESTED IMPROVEMENT IX BINOCULAR MICROSCOPES. 375 



I have some photographs kindly taken for me by Mr. 

 Taverner showing this.* This equality of magnification of 

 different layers has been definitely stated as a fact by Abbe 

 in his paper, " On the Mode of Vision with Objectives of Wide 

 Aperture," t but I have not come across any explanation. The 

 full explanation of this curious matter is not altogether clear, % 

 but one or two contributory causes are evident. The first one 

 is that, although separate layers form images of different size 

 at the respective planes at which they are in true focus, when 

 we refer them to a common plane we are dealing with their 

 projections on that plane ; Fig. 4 illustrates this. AA, BB, CO 

 have quite different magnifications at their own respective image 

 planes ; but in the plane A the axis of the diffusion discs of the 

 points B and practically coincide in position with A. 



The other reason seems to be that the depth of the different 

 layers which can be seen at one time is so small compared with 

 the distance of the object from the objective that there is little 

 scope for any appreciable perspective effect. It is as if we 

 expected to notice any perceptible difference in size between the 

 front and back of a little square with 1-in. sides held at a 

 distance, say a yard, straight in front of the eye. To further 

 complete the comparison, we may imagine the front to be seen 

 sharply, but the back to be seen more hazily, so that its precise 

 length could not be judged quite so clearly. 



* Certain photographs, as yet unexplained, even show a tendency for the 

 farther set of lines to be imaged slightly wider apart than the nearer set ; 

 in no case were the latter set the wider. 



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



J Since this paper was read these statements require modification. Thanks 

 to Mr. A. E. Conrady, who drew attention to the matter in the discussion 

 following the paper, and likewise to Dr. A. Kohier, of Jena, to whom I am 

 indebted for a long explanatory letter, it has been pointed out that the 

 question was dealt with by Abbe in a paper in which one would scarcely 

 seek for it. In an article, ; ' Ueber mikrometrische Messung mittelst 

 optischer Bilder" (vide Abbe's Collected Works, pp. 1G5— 172), it is shown 

 that the magnification of different layers of the object in a given plane of 

 the eye-piece is dependent on and controlled by the position of the stops 

 above the objective. It is not possible here to enter in any detail on this 

 somewhat difficult subject. Suffice it to say that Abbe demonstrates that 

 by having a stop in the upper focal plane of the objective, thereby 

 producing what he has termed a " telecentric " passage of the light ray>. 

 the magnification of the various planes of the object can be practically 

 equalised in the plane of the eye-piece diaphragm. 



Journ. Q. M. C, Series II.— No. 59. 27 



