ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 231 



Microscope by the stereo-photomicrographs taken by Mr. H. Taverner, by 

 means of excentric circular stops placed behind the objective, Mr. 

 Rheinberg saw that the same method might be applied to binocular 

 microscopes. His justification for this is given in the following extract 

 from the paper — 



" If parallactic displacements of out- of- true-focus layers of the ob- 

 ject constitute the mechanism by which stereoscopic effect is produced, 

 this in itself furnishes the necessary proof that the whole image, barring 

 the one plane in true focus, consists of diffusion disks. The size and 

 shape of these diffusion disks is therefore an important matter. We 

 saw in the first part of this paper that the size of the diffusion disk 

 varies directly as the size of the portion of the objective used ; further, 

 we saw that the shape it assumes is the same as that of the portion of 

 the objective utilised. It is evident, therefore, that to have pictures of 

 maximum clearness it is desirable to have these disks as small as tht> 

 circumstances permit, and also that they should be circular in shap. 

 At present, in binocular Microscopes no regard is paid to either of thes' 

 matters ; the size of the diffusion disks is not adapted according to the 

 depth of the object to be viewed, and the image is formed of over- 

 lapping disks semicircular in shape. An unsymmetrical shape like this 

 results in the image of the same object being less distinct in certain 

 directions than in others, or varying in distinctness according to the 

 position in which it happens to lie in the field. 



" How it has come about that these matters have been overlooked 

 is simple enough. As regards Microscope images, attention has been 

 chiefly concentrated — and justly so — on the perfection of the image of 

 the object layer in true and perfect focus in the view plane, and for this 

 particular plane other conditions prevail. It is the one layer which is 

 free from parallactic displacement, no matter which part of the ob- 

 jective may be used. It is also the layer for which the laws framed 

 from the study of the diffraction of light apply more particularly. 

 And one of these laws is that the 'diffraction disks,' of which the 

 image in this plane is composed, vary inversely in size with the aper- 

 ture of the objective (or of the part of the objective) utilised. Smaller 

 disks mean greater resolving power so long as the image magnification 

 remains unaltered ; therefore, for this one plane, the larger the aperture 

 of the objective employed, the better the images, and the largest aperture 

 available in binoculars is the half-objective. An instructive experiment 

 consists in viewing a Grayson band plate with a binocular Microscope. 

 The effect of the semicircular shape of the half -objective may then be 

 shown by rotating the plate. When the rulings lie in the direction 

 of the straight edge of the half-objective, a band with only about 

 half the number of lines per inch is resolved as when they lie in the 

 direction at right angles to this. 



" Although within certain limits the same principles hold good with 

 respect to slightly out-of-focus layers, the general feature remains that 

 diametrically opposite conditions apply, as regards diminishing the size 

 of the disks, when the layer of the object is in true focus and when it 

 is not. The one necessitates the employment of parts of the objective 

 aperture as large as possible ; the other requires them to be as small as 

 possible. 



