108 COKRESPONDENCE. 



of University Street, who suggested that a covering of copper would 

 be the best protection. 



It will be seen by this arrangement that the whole of the chimney 

 is covered with the exception of a small space through which the 

 light passes, so that it acts as an ordinary chimney-silver-reflector and 

 lamp shade, and in one. 



Thus saving much time, trouble, expense, and space. 



It is supplied by Mr. Swift at the trifling cost of fifteen pence. 



I remain, yours &c., 



Fkederick Blankley. 



How TO Select Spectacles for Near-sight and Far-sight. 



Rochefort-sue-Mer. 



Sib, — I take the liberty of sending you a note, which you will 

 please insert in the ' Monthly Microscopical Journal,' if you think it 

 suitable. 



I am. Sir, yours, 



MOUCHET. 



Note concerning certain Micrograpkical Measures, and containing some 

 information useful to persons suffering from Myopia and Presbyopia. 



I do not know the instrument employed for measuring the thick- 

 ness of the thin glass plates used for covering microscopic objects, nor 

 do I know if there is one for valuing the relief of those objects. In 

 every case, I use my microscope for both pui'poses, and also for point- 

 ing out to sufferers from Myopia and Presbyopia the glasses suitable 

 to their sight ; and the object of the present note is to describe the 

 process that I employ. 



In every microscope of a certain price there is, besides the tube 

 which is worked by friction, or by a rack movement, or by both, a 

 micrometric screw for finishing the focussing, that is, one with a very 

 fine movement, and consequently possessing great delicacy. It is 

 important to have this screw made with gi-eat care. This screw is 

 what I use for measuring the thickness of covering glasses, and of 

 microscopical objects. For this purpose I have fixed on its head, 

 which, as in most instruments of this kind, is placed in the upper part 

 of the tube,* a dial divided into 100 parts, and high enough above the 

 m.oleiee part not to impede the movement of the fingers, when focussing. 

 On the canon which supports the tube is a ring, slit like the canon 

 itself, and kept in its place by its own elasticity. This ring carries 

 an index opposite the dial, and can be turned so as to bring back the 



* In this respect differing from English instruments. 



