MEMORANDA. 1G3 



great advantage over a clear aperture of 7V on the same 

 object mounted dry ; because, if it is really as I have stated, 

 that the visibility of the markings depends upon their opacity, 

 we shall, in the former case, not only obtain more light 

 through the object itself, but there will also be no appreciable 

 loss from the reflection of the containing glass surfaces, as 

 there is in a dry mounting. 



To those who may still doubt the question of the loss of 

 aperture, on objects immersed in refractive media, I will 

 recommend a trial of the following simple and instructive 

 experiment, bearing reference to the point in question. 



Spread some moistened Diatomacece on a glass slip, and 

 when dry cover the deposit with a piece of thin glass ; place 

 the slide under the microscope, and with an eighth object- 

 glass, select a specimen that may be considered as a test, 

 adjust and focus carefully, and so let it remain. Next, take 

 a drop of oil, or thin Canada balsam, at the end of a wire, 

 and drop it on to the edge of the tliin glass cover. If the slide 

 is warm this will rapidly insinuate itself between the glasses. 

 Look through the microscope, and wait for its passage across 

 the field of view ; directly that it has passed, it will be found 

 that everything that was visible on the slide, the moment 

 before, has vanished from sight ; because the oil, or balsam, 

 has caused a different refraction of the rays from the object- 

 glass, the focus of which has, in fact, become lengthened ; 

 for, in order to bring the object again into view, the objective 

 must be brought back a farther distance from the test; it will 

 then be found, if this was at all difficult, that the stria? are 

 now totally invisible, under the same conditions of light, and 

 it will require a different arrangement of illumination to bring 

 them out again, together with an alteration in the adjustment 

 of the object-glass. It may be supposed that these effects are 

 analogous to those which would be produced by the interpo- 

 sition of an extra thickness of glass ; but just the same plie- 

 nomena are observed if the objects are adherent to the thin 

 cover itself. 



With this example I conclude my remarks. I consider 

 that the main point, that balsam or fluid mounting does 

 create a diminished aperture, still remains an untouched fact. 



The inquiry is one of considerable interest ; and, as it may 

 end in an application of much utility, should be prosecuted 

 farther. — F. H. Wenham. 



On the Klea^iturcineut of the Aperture of Objectives, In yOUr 



last number I see that Mr. Sollitt proposes a new method 

 for measuring the apertures of objectives, in discussing which 



M 2 



