MEMORANDA. 241 



greater will be any dissimilarity in colour of the various parts ; 

 but if you destroy the intensity of the light, by using a high 

 power eye-piece, you might as well try to see it with the low 

 power, and a telescopic sun-shade over it. I have a five-feet 

 achromatic telescope which will show the fifth star in the 

 trapezium of Orion very well with a power of 100 ; but with 

 higher powers you cannot see the small star, because the 

 telescope has not sufficient light to show the dift'erence in the 

 colour of the faint star and the nebula by which it is sur- 

 rounded. This equally applies to microscopic vision, par- 

 ticularly where the object is very transparent, and the difference 

 in colour between the object and the balsam comparatively 

 small. 



With regard to the diminished aperture, as made apparent 

 by the methods employed by Mr. Wenham and Dr. Robinson, 

 when applied to balsam-mounted objects, I think it is very 

 easy to account for the conclusions they have been led to ; 

 for every one conversant with optical instruments knows 

 that the larger the aperture in proportion to the focus, the 

 greater will be the aberration of the rays passing through or 

 from the edge of such aperture, as, in a telescope of large 

 aperture, one angular inch in tlie centre will give as much 

 light as eight or ten angular incl;es taken in the form of a 

 ring round the extreme edge of the glass ; no wonder then, that 

 in the objective of a microscope, where the diameter of the 

 aperture is seven and a lalf times that of the focus (which it 

 is when the aperture is 152°), the aberrations from near the 

 edges of the glass should be so' great as to cause the rays not 

 to be visible after having passed through balsam. 



I recollect that the last time I had the pleasure of seeing 

 Mr. Wenham he told me he had made a l-8th, the aperture 

 of which was somewhere between 170° and 180°, but on 

 account of tb.e weakness of the rays at the edges, arising from 

 aberration, he would not undertake to say within 4° or 5° what 

 the exact aperture really was. 



With these observations I shall conclude my remarks, being 

 fully persuaded that when objects are mounted in balsam that 

 medium has no effect in reducing the aperture of the objective, 

 and that no extei'nal cause, except a fluid or other medium 

 in actual contact with the objective, can, consistently with 

 the known laws of optics, produce such an effect. — J. D. 

 SOLLITT, Hull. 



On IVashing and Concentrating Diatoiuact ae. — Having read in 



your last Journal the excellent paper by Mr. Okeden " On a 

 mode of Washing and Concentrating Diatomaceous Earths," 

 VOL, HI. R 



