ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 509 



" In the preceding experiments with the binoculars, low power and 

 coarse objects were used ; the following, however, were made with 

 diatoms having finer details requiring higher powers. First a P. angul- 

 atum (dry) was set up under a monocular with an achromatic ^ with a 

 correction collar and C eye-piece, a dry condenser axial light, cone 

 about ^, no stop or slot. A good image was obtained, the dots were 

 distinct and clear, with no tendency to run into lines in any direction, 

 which may be taken as evidence of a good lens and good definition. The 

 Wenham prism was now pushed in, and the image went to pieces 

 directly. This might have been expected for the reason previously 

 given, viz. that half the horizontal aperture of the objective had been 

 cut off by the prism ; the lens had an aperture of 0'<S5 N.A., the half of 

 which is 0*43, an amount insufficient to resolve the P. anguJatum. So 

 the conditions were that there was ample aperture for resolution in the 

 vertical, but insufficient in the horizontal direction, and the image showed 

 it. A coarser object, P.formosum^ in balsam, was then tried with the 

 same lens in the same manner. Under the monocular the image was 

 bright and distinct, the dots were nice and black, and the silex white, 

 without any running into lines or blurring of any sort. When the prism 

 was pushed home the image was (to use a modern term) rotten. The 

 best thing that could be done was to equalize the error by placing the 

 directions of the rows of dots about 45° to the vertical. Even when this 

 was done the image was very poor. 



" As the experiments with these comparatively coarse details failed, 

 they were not pushed further on more difficult objects. These objects 

 under the parallel-tube binocular were not mutilated to the same extent 

 as they had been under the Wenham. There was a slight loss of sharp- 

 ness, but the dots were there and did not run into lines. With high- 

 power images with this microscope the better plan is to set the tubes for 

 full orthostereoscopic vision, and then, when the eye has realized the 

 form of the object, to increase the interpupiliary distance, and, although 

 there will be less orthostereoscopism, the mind will hold the form of the 

 object. These experiments have not altered the opinion I have held for 

 many years with regard to the Wenham binocular, viz. ■ that it is a 

 valuable instrument ; it is working at its best with the most useful 

 lenses, viz. |, and low-angled |-inch. With powers lower than the 

 1-in. the orthostereoscopic effect decreases ; its upper useful limit seems 

 to be reached (only so far as mj own experience and instrument go) 

 with a ^ of 0-65 X.A. 



" Either ' N. E. B.'s ' microscope differs from mine, or he has some 

 way of working it with which I am not acquainted. I would very 

 much like to meet him some day and compare notes. I quite agree with 

 him with regard to the beauty of the Aulisci. I was the first to 

 discover the fine structure in the processes and the first to photograph 

 it. The photograph used to hang on the wall at the R.M.S., but it may 

 not be there now. Grevill's Aidiscus under a | on a dark ground is 

 indeed a most beautiful object. It is like a sapphire brooch with 

 diamond points ; and, as ' N. E. B.' says, there are many very beautiful 

 forms in this genus. 



" If ' N. E. B.' wants something quite new, if he will examine an 



