1909.] NATURAL SCIENCES OF PHILADELPHIA. 187 



greater than is commonly found between that of two objectives of 

 identical construction from the same maker. Right here it may be 

 well to mention two other fallacies regarding the Wenham binocular 

 which seem likely to be perpetuated in microscopical literature, having 

 even found a place in Spitta's recent valuable and generally accurate 

 work on the microscope. One of these is, that the beam passing up 

 the left tube produces a larger image (unless this is count aacted in 

 the ocular) because it has to travel farther on account of the two 

 reflections in the prism. As I many years ago explained, the path of 

 the beam through the prism is but slightly over one and a half times 

 the length of the path of direct beam alongside the prism. As this 

 distance is in glass of at least 1.50 R. I. the ray emerges from the prism 

 as if it had traveled only two-thirds as far in air. and the optical tube 

 length is practically identical in both tubes, the difference not reaching 

 a millimeter with an ordinary well proportioned prism. The other 

 fallacy is that such binoculars are not suited for higher powers than 

 about a half inch, while they really work well with objectives up to an 

 eighth inch or higher, if the latter are mounted, as Wenham himself 

 recommended, in short mounts, so as to avoid parallax between back 

 lens and prism. If anyone doubts the immense advantage of stereo- 

 scopic effect with high powers, let him attempt with a monocular to 

 demonstrate the anatomy, particularly the position of the unextruded 

 proboscis, in a glycerine-mounted specimen of the motile condition of 

 our too common pest, the San Jose scale, which I have never seen cor- 

 rectly figured, and then note the difference when a binocular is em- 

 ployed. The sculpturing on spores and capillitium of Myxomycetes, 

 spores of ferns, etc., afford other good tests of its efficiency. 



Returning to diffraction phenomena, I almost hesitate to refer to 

 the two experiments by which Wright seems to think he has demolished 

 the Abbe theory, as their fallacy is so self-evident that it must have 

 been at once recognized by every microscopist having more than the 

 most elementary acquaintance with the subject. The assumption, in 

 the first experiment, that when the aperture of objective is fully 

 illuminated there are no diffracted rays present, because they cannot 

 be seen, is absurd, and needs no further consideration; but anyone 

 desiring to know just what is the character of the diffracted beams 

 resulting from any particular structure under the conditions specified, 

 may secure the information by employing a pinhole diaphragm in a 

 freely moving carrier connected with substage condenser. A circle 

 should be drawn on a piece of paper to represent back of objective, 

 and the diffracted beams visible plotted in with a red and blue pencil. 



