TEST OBJECTS. Ill 



order of their refrangibility, by putting the object both within 

 and without the focus, as well as by viewing it at the focal point 



Spherical aberration is most sensibly felt in viewing opaque 

 objects, especially if of the brilliant class. It shows itself in 

 a variety of ways : first, as a diffused nebulosity over the whole 

 field of view; secondly, as a confined nebulosity, extending 

 only to a certain distance from the object; and thirdly, in a 

 want of sharpness and decision in the outline caused by a 

 penumbra or double image, which can never be made to lap 

 perfectly over the stronger or true one. Destitution of spheri- 

 cal aberration is evinced by the absence of these appearances, 

 and by the vanishing of the image immediately that the object 

 is put out of focus either way. 



To ascertain the defects alluded to above, a minute globule 

 of mercury on a black ground, known as an " artificial star," 

 is used. It presents a very minute point of light. Very mi- 

 nute globules of mercury, spread over a blackened surface, are 

 viewed as opaque objects, being illuminated by the light from 

 a window or lamp thrown on them by a condensing lens. 

 When one of these globules is in the focus of a single lens 

 object-glass, a strong coma surrounds the miniature image of 

 the window seen in the globule, and when within or without 

 the focus, the light of the window swells out into a circular 

 disc. These appearances are more or less accompanied by 

 prismatic colors. 



When an achromatic combination, perfectly corrected for 

 both kinds of aberration, is employed, the globule should ex- 

 hibit similar appearances both within and without the best 

 focus; and when at the focus, the point of light should be seen 

 as a minute disc, free from irradiations and color, except a 

 general blueness, which results from the irrationality of the 

 spectra of the different glasses of which the object-glass is 

 composed. 



