On the Achromatic Microscope. 



265 



Art. VI. — On the Achromatic Microscope; for the Journal of Sci- 

 ence, by Edward Thomas. 



In a former article on improvements in the microscope, .published 

 in Vol. XIX. of the Journal of Science, a description was given of 

 the best form of the achromatic microscope then adopted. Material 

 improvements have since been made, which it is the object of this 

 paper to explain. Many of these have been introduced by Alden 

 Allen. 



When a lens is required with a focal distance of an inch or more, 

 and is to be used as a simple microscope, one single lens of plate 

 glass, having its radii nearly as 1 to 6, will exhibit objects about as 

 distinctly as one of any other construction. But when the focal dis- 

 tance is half an inch or less, an achromatic lens is superior, whether 

 for simple or compound microscopes. The form represented in 

 Fig. 1, is the best for simple microscopes, when the focal distance is 

 from one half to one fourth of an inch. If a shorter focal distance 

 be required, and a large aperture is necessary, it will be most advan- 

 tageous to employ the sextuple achromatic shown in Fig. 2. 



In Fig. 1, is shown the best form of the object glass of a compound 

 microscope, when an aperture is required nearly equal to one half of 

 the focal distance : a is the first surface, b the last, and o the place 

 of the object to be viewed. The following are the dimensions and 

 radii.* 



Inch. 



Radius of the 1st surface, 

 do. 2d do. 



do. 3d do 



do. 4th do 



do. 5th do 



do. 6th do 



o:2o]p^"^^ 



0.15 \ ^"^'• 

 0.15) , ^ 

 0.20 \ P^^^^^ 



Focus of the compound lens, 0.27 

 Diameter, . • . . . 0.14 

 Clear aperture, . . 0.10 to 0.12 



* The. fic'uies arc all drawn four limes the real size. 



