58 Improvements 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 in 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. 



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, b at the last, and O represents the 

 place of the object to be viewed. 



Inch 



Radius of 1st. surface, 0.44 



do. 2d. do. 0.26 $V m *- Fig. 1. Fig. 2 



a 



b 



do. 3d. do. 0.26 

 do. 4th. do. 0.44 



do. 5th. do. 0.44 

 do. 6th. do. 0.70 



do. 7th. do. 0.174 

 do. 8th. do. 0.26 



do. 9th. do. 0.26 > fl . O O 



do. 10th. do. 0.44 >mnl ' 



do. 11th. do. 0.44 ) j 

 do. 12th. . do. plane. ) P 



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 that 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. 



