MEMORANDA. (£lo) / d J 



either turned aside or elevated as before, to move the slide to 

 and fro, either by hand or stage movements, until the spot of 

 light from the condenser indicates the spot at which the 

 vertical and horizontal lines beyond the margin of the cover 

 would intersect each other, if produced. 



Of course, upon the accuracy with Avhich the bearings have 

 been taken will depend the facility of finding an object. But 

 with ordinary care and a tolerably true eye, there is no diffi- 

 culty. 



The following diagram will show the mode of registry, and 

 how it may be applied to any number of objects on the same 

 slide — 



The dots, it is almost unnecessary to remark, are appended 

 merely with a view to indicate the points at which the objects 

 to be registered occur. — G. L. "Wallich. 



Improvement of the Camera Lucida. — One of M. Nachet's 

 ingenious applications of the prism to the microscope fur- 

 nishes a hint for the improvement of the camera lucida, 

 which 1 desire to bring under the notice of yourself and your 

 readers. I refer to the arrangement described and figured 

 on p. 706 of the second edition of Dr. Carpenter's work on 

 the microscope. A prism of peculiar form is there seen, ap- 

 plied as a camera lucida to a vertical microscope. 



To the arrangement in question, as a whole, I do not attach 

 much importance; for, first, our English microscopes are 

 generally of too tall a build to admit of being at all commo- 

 diously used in a vertical position for any length of time ; 

 and, secondly, if they could be commodiously so used, the 

 stage would be in the way of the hand ; while, moreover, the 

 paper not being in the place where it seems to be, but away 

 in front of the instrument, I venture to think that this would 

 seriously interfere with the free use of the pencil in tracing 

 the image. 



It is to a small adjunct of M. Nachet's prism that I 

 refer, as holding out a prospect of advantage ; I mean the 

 piece, marked e, in Dr. Carpenter's figure. It is well known 

 that many — perhaps most — microscopists find considerable 



