compound lenses and object-glasses. 247 
to which the lens is to be applied must decide. In a burning 
glass it is of the highest importance. A slight consideration 
will suffice to show, that the difference of temperatures pro- 
duced in the foci of a double convex lens of equal radii, and 
one of the same focal length but of the best form, must be 
very considerable. In order to try whether even the latter 
might not be improved by the shortening of the focus, and 
the superior concentration of the exterior rays, by applying 
a correcting lens of one of the forms above calculated, in 
spite of the loss of heat in passing through a second glass, 
I procured two lenses to be figured to the radii assigned 
in the first column of the foregoing table. They were 
about three inches in aperture, and when combined as above 
directed, the aberration was almost totally destroyed, and 
probably would have been so completely, had the index of 
refraction proper to the glass been employed, instead of 
that adopted in our calculation for brevity. Their com- 
bined effect as a burning lens appeared to me decidedly 
superior to that of the first lens used alone, and there is there- 
fore good reason to presume that the effect of the other con- 
struction which, with the same loss of heat, affords a much 
greater contraction of the focus would be still better, and I 
regret not having tried it in preference. 
14. In eye-glasses and magnifiers, if we would examine a 
minute object with much attention, as a small insect, or (wffien 
applied to astronomical purposes) if we would scrutinize the 
appearance of a planet, a lunar mountain, the nucleus of a 
comet, or a close double star, where extent of field is of less 
consequence than perfect distinctness in the central point, too 
much pains cannot be taken in destroying the central aberra- 
