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 
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 5 and in a greater frecdom 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 
it is shortened.* These advantages more than compensate for the 
loss of light, caused ‘by a greater number of reflecting wae: 
The following are its dimensions and radii. 
ee dick of the Ist meee . 10 | 
a od ar plate. 
ter ee ae Pig. 2 
do. 4th do. 0.20 ' ke | 
do." “Sth “do. “0.20 
do. = 6th. do. «0.267 plate. 
~ do. 7th do. 0.15 2, 
do. “Sth. dow eae. ¢ Pile: 
do. 9h do. 0.15 24. 
do. 10th do. 0.15 * in : 
do. S'skithe de, s 8.6 ? S 
do. 12th do. 0,20 plate. 
Focus of the compound lens, 0.17 
. Diameter, . ee Gc eee 0.15 
Clear aperture, “i ee 13 to 0.15 
a Fig. 3 exhibits the form of the microscope to be deat 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. 
ninemsn 
ae 
* “The former sextuple combination was free from spherical aberration, only wher 
the tube was extended toa certain len ngth. 
t All the surfaces in contact are cemented in each of the three combinations 
