270 SUPPLEMENT TO 



the focus will resemble figure 9, plate 13,* with the addi- 

 tion of a luminous spot in the middle ; that without it 

 figure 7 /f ) and the focal one figure 7' ' In this case 

 parallel rays are of course employed, and the aberration of 

 the lens will be greater with them than with diverging 

 ones ; but the image of a radiant point formed by a solar 

 microscope, with conjugate foci, of the same length as in 

 an engiscope, is a case in point, and will be found to pre- 

 sent a very similar result. 



The following are observations made upon the chro- 

 matic and spherical aberration of a plano-convex lens of 

 nine-tenths of an inch focus and four-tenths of an inch 

 aperture, with its plane side towards the radiant, employed 

 as the object-glass of an engiscope, having an achromatic 

 Huyghenian eye-piece. The objects used were, first a 

 piece of watch-dial plate, with figures enamelled white, 

 upon a black ground, see figure 5' ; and secondly, artifi- 

 cial stars formed by crushing a globule of mercury with a 

 piece of iron on a slip of black glass, so as to form very 

 minute spheres of quicksilver, reflecting the light of the 

 sun, a candle, or a window, at the pleasure of the observer, 

 (see figures 5, 6, and 6'.) 



SPHERICAL ABERRATION. 



1st. Dial Plate. There is a strong nebulosity diffusing 

 itself around the borders of the white figures, and extend- 

 ing to a considerable distance from them, so that they are 

 very ill defined, (see figure 7 /// )> when put out of focus, 

 the following phenomena present themselves. The prin- 



* This is the original reference as publishtd in the " Cabinet." 



