58 Improve mejrts in the Microscope. 



the place of the object. The surfaces in contact are cemented with 

 Canada balsam, so that the compound lens has only four reflecting 

 surfaces, and m consequence the vision is much clearer. The quin- 

 tuple object-glass has a larger field of distinct vision than any other 

 achromatic combination that I have seen. 



I have a sextuple object-glass. Fig. 2, shown in section of twice 

 its real size, that consists of two double-concave flint lenses, and four 

 convex lenses of plate glass. Its dimensions and radii are as fol- 

 lows ; a stands at the first surface, h at the last, and O represents the 

 place of the object to be viewed. 



Fig. 1. Fig. 2. 



o o 



Focus of compound lens, 0.25 of an inch. 

 Total diameter, . . . 0.25 do. 

 Clear aperture, . . . 0.20 do. 

 Specific gravity of flint glass =3.457. 



The sextuple object-glass bears a larger aperture, in proportion to 

 its focal distance, than any other that I have used ; and the field of 

 distinct vision is greater than it is in any other achromatic combina- 

 tion tliat I have examined, except the quintuple lens. It is cemen- 

 ted so as to have only four reflecting surfaces. Upon the whole I 

 consider it as the best combination known at present. 



The focal distance of the compound lens. Fig. 2, is one fourth of 

 an inch ; but when the lens is used as the object-glass of a compound 

 microscope, which magnifies two hundred or three hundred times 

 the diameter, it will show as minute an object, as can be seen with a 

 good single lens of mine that has a focal distance of 0.014 of an 

 inch, and which magnifies the diameter five hundred times. 



