166 MEMORANDA. 



finer and fainter. If Mr. Sollitt will repeat one of my ex- 

 periments, select a single valve, whose details are barely 

 visible when dry (the objective may be of moderate aperture i, 

 successively introduce under its cover, water, alcohol, and 

 balsam, and use very deep eye-pieces, which are the sure tests 

 of deficient light, he will probably find reason to change his 

 present opinion as to the effect of dense media. 



It may be of use to some of your readers to know that I have 

 ascertained the nature of the irregularity which injured 

 the circumference of the objective described in my paper as 

 No. 4. (Quarterly Journal of Microscopical Science, vol. ii. 

 p. 296.) I sent it to a distinguished optician, Mr. Grubb, of 

 Dublin, to have a graduation put to its compensation, who, on 

 taking it asunder, found that a little of the cement which 

 unites the lenses was visible round the edge of one: this he 

 very cautiously removed I was struck with tlie improved 

 performance of the objective, but referred it to the superior 

 action of a new microscope which he had made for me, till I 

 happened to use it for the purpose of shewing the nature of 

 this defect to a friend. To my surprise it was gone, and the 

 disc of light was unbroken to the edge, giving 129° instead of 

 102°, which had been the really effective part of the aperture. 

 I do not pretend to say that the same occurred in the others 

 which I examined (in No. 6 the case was certainly different) ; 

 but it is, at least, desirable that when such a defect is found, 

 this probability should be kept in mind.* J. R. Robinson, 

 D. D., Armagh. 



On an iinproved Fiiitler for the Microscope. — The accompanying 



plan for a Jinder, I have used for some time, and as I have 

 found it to work very well, and as it has moreover been seen 

 and approved of by two or three veteran microscopists, I have 

 ventured to send a description of it to your Journal. 



The drawing I send is a view of Ross's stage to his best 

 microscope, which is the instrument I use, and to which I 

 have adapted the finder. 



* This microscope deserves to be known : among other vahiable im- 

 provements it has anticipated, and in a better form, one proposed in your 

 last numljer, as " a new achromatic condenser." Mr. Grubb "s ilkmiinator 

 is a prism whose aberrations are corrected for a lamp placed at a given 

 distance in the plane of the stage. It travels on a graduated arc of 120", 

 and through this range its focus continues on the object, sufficiently bright 

 for the highest powers. I have used it at 50" with 3200, and find the 

 power of examining tissues at various obliquities very useful. If raised 

 above the stage it gives at once a capital illumination for opaque objects ; 

 it acts well with Licberkuhn and Nicol's prism, and trifling additions 

 make it equally effective witli Mr. Bergin's parallel illuminator, which 

 shows some objects with peculiar distinctness. 



