.MEMORANDA. 135 



looking, with an instantaneous touch of tlie finger you slily 

 pop in the prism. "Now, how does it look?" he will pro- 

 bably say. " Oh ! just the same ; unless that I think you 

 have slightly altered the light." "True; but you see every 

 part of the object as well defined as before ?" " Yes, quite 

 as well ; and I should say even more agreeably, for I fancy 

 there is not quite such a glare of light." " Ah ! then you 

 will not readily believe that I have actually cut off one half 

 of the entire disc of the object-glass." " You ddn^t say 

 so !" " Perfectly true, however." The next step is to remove 

 the opaque disc, and, for the first time in his life, submit to 

 his astonished gaze — binocular vision! the double ray 

 uniting in the cerebrum, to form one distinct and beautiful 

 image, exhibited, moreover, with the most marvellous ste- 

 reoscopic effect ! 



I assure you it actually compels people to sho7U with 

 amazement ! ! " Well, I never beheld anything equal to 

 •that ! It is most magnificent ! I seem to see part behind, 

 part in real perspective !" (This effect, by the way, is ad- 

 mirably shown by a good specimen of hypersthene, with a 

 1-inch objective and Lieberkuhn reflector.) Now, the best 

 part of the practical joke remains. After allowing your 

 friend to luxuriate for some time over this gorgeous spec- 

 tacle, and while he is still earnestly gazing at it, you sud- 

 denly withdraw the prism ; when he will probably as sud- 

 denly withdraw his head, exclaiming, " Dear me ! how is 

 this? Why, I appear suddenly to have lost half my eye- 

 sight. How very unpleasant ! What have you done ?" 

 " Done, sir ; why, I have merely brought back the micro- 

 scope suddenly to its ordinary state. Can you, now, believe 

 it? — that is really the way you have been using the instru- 

 ment all your life !" 



And now a few parting words on another topic. My last 

 communication on the above subject was, by some trifling- 

 editorial inadvertence, I suppose, headed " Further Notes 

 on Finders;" which, I have been informed, puzzled a good 

 many. But I must take the present opportunity of confess- 

 ing my error in supposing myself like Columbus (No. 

 xxxii, p. 201), for I have since had the mortification of 

 finding myself forestalled; for the "double nose-piece" is 

 recommended as a finder by Dr. Carpenter, in his excellent 

 work, 'The Microscope and its Revelations,^ paragraph 51. 

 I was utterly unaware of this when I sent the communica- 

 tion. No. xxxii, p. 198. But I do not regret having done 

 so, as it has been the means of drawing the attention of 

 many to the subject. Moreover, every microscopist may 



