184 



MICROSCOPIC OBJECTS. 



apparatus which can be apphed to this microscope, such 

 as condensers, micrometers, dissecting instruments, camera 

 lucida, micrometer and erecting eye-pieces, &c., as they 

 are fully treated upon in the works before referred to. 

 It may not, however, be amiss to mention that the cost 

 of this Microscope varies greatly, according to the quan- 

 tity of apparatus attached to it, and the number of object- 

 glasses. When plainly equipped, the price is from twelve 

 to fifteen guineas ; and as additional lenses and apparatus 

 can be applied at any time as the necessity for them 

 arises, it is often better in the outset to purchase the 

 instrument in its simplest form, rather than encumber 

 oneself with a quantity of apparatus and glasses which 

 may never be required in practice. 



Table of the average Magnifying Powers of the Standard Achro- 

 matic Microscope, with complete sets of Object-glasses, given 

 in diameters, and calculated by a ten-inch sight. 



OBJECT-GLASSES. 



Focal lengths. A. 



One inch 30 



Half an inch 60 



One-third of an inch 90 



One-seventh of an inch 200 



One-tenth of an inch 250 



One-twelfth of an inch 350 



One-twentieth of an inch 500 



Note. — The above calculations are given from Mr. 

 Pritchard's list of powers, furnished with his microscopes. 

 They are much below those usually stated by opticians. 

 Thus the power ** 300 diameters" in the above table is 

 commonly called 400 linear or 16,000 superficial. 



Method of using Micrometer Eye Pieces. — First, select 



V*; 



