DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 99 



sons confine their attention to a particular class of objects, 

 some little consideration is necessary for selecting those 

 sets which are best adapted to the purpose. It is by no 

 means an unfrequent occurrence, that applications are 

 made to me, by those who have purchased expensive 

 instruments, for some particular object-glass, which they 

 do not possess, whilst they have others which to them 

 are of little or no value. 



The first column in the following table gives the side- 

 real focal length in inches and parts. This focal distance, 

 in shallow object-glasses, is less than the distance an 

 object should be placed at from the lens when adjusted 

 for distinct vision ; whereas, with the higher powers, the 

 sidereal focal distance is greater than that between the 

 object and the lens. It varies also with the length of the 

 body. The second column shews the distance at which 

 the object should be placed from the object-glass. The 

 third column gives the mean angle of aperture, which 

 varies a few degrees either way. The last column gives 

 the magnifying powers in diameters, in combination 

 with different eye-pieces. It is calculated by a 10-inch 

 standard for sight. 



M6th . . 1.40th. .70 450 to 3000 



