266 



On the Achromatic Microscope, 



Fig. 2 represents the best form of the sextuple object glass, and 

 is to be used when an aperture nearly as great as the focal distance 

 is requisite. The superiority which this form of the sextuple lens 

 possesses over the one formerly described, consists in its greater sim- 

 plicity, having a less variety of curves ; in its affording a greater field 

 of distinct vision ; and in a greater freedom from secondary aberra- 

 tion of figure, when the aperture in each bears the same proportion 

 to the focal distance. It is also practically free from spherical aber- 

 ration, both when the tube of the microscope is lengthened and when 

 it is shortened.* These advantages more than compensate for the 

 loss of light, caused by a greater number of reflecting surfaces.^ 

 The following are its dimensions and radii. 



Radius of the 1st surface. 



do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 



2d 



3d 



4th 



6th 



6th 



7th 



8th 



9th 



10th 



11th 



12th 



do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 

 do. 



Inch. 



0.19 ). , , 

 0.267 5 P^^^®' 

 0.20 

 0.20 



0^267 5 P^^^^° 



0.15 



0.20 



0.15 



0.15 



0.15 



0.20 



I flint. 



] 



> plate » 

 I flint. 



> plate. 



Fig. 2. 



Focus of the compound lens, 0.17 



Diameter, 0.15 



Clear aperture, .. 0.13 to 0.15 



Fig. 3 exhibits the form of the microscope to be adopted when an 

 aperture greater than the focal distance is necessary. It consists of 

 nine lenses, of which six are plate glass and three flint glass. The 

 following are the dimensions and radii. 



* The former sextuple combination was free from splicrical aberration, only when 

 tlie tube was extended to a certain length. 



f All the surfacea in contact are cemented in each of the llucc combinations, 



