250 SUPPLEMENT TO THE 



powers of telescopes have been separated into two parts, 

 and penetrating power considered distinct, from a power 

 of shewing objects of difficult definition. I have acted in 

 the same spirit relative to the less important instruments. 

 Thus, a telescope, having a very large aperture, though of 

 an erroneous figure, will shew nebulae, and clusters of stars, 

 totally invisible by a small telescope of the most perfect 

 kind ; and the latter again will, in its turn, shew double 

 stars and other minutiae, which the former would be utterly 

 incompetent to render manifest. In the same manner, an 

 engiscope of a small angle of aperture will define, and 

 shew very exactly, an enamelled dial plate, the outline of 

 the brilliant scales of a variety of beetles, animalcules, and 

 an innumerable quantity of common objects, too tedious 

 to mention ; but it will utterly fail upon the lined objects ; 

 while another, having a large angle of aperture, though 

 replete with aberration, will nevertheless shew many of 

 these, (though not so well, of course, as if aplanatic and 

 achromatic ;) while, if tried, on a dial plate, or a piece of 

 diamond beetle, &c. it will exhibit its imperfections in a 

 most glaring manner, and shew a total deficiency of what 

 I call defining power. There are objects which at once 

 serve to exhibit the perfection of a microscope, or engiscope, 

 both as to defining and penetrating power, provided the 

 manner in which they are shewn is duly attended to. 



The following aphorisms will serve to express my views 

 relative to the subjects of this memoir. 



1. A test is an object which serves to render sensible 

 both the perfection and imperfection of an instrument, as 

 to defining and penetrating power. 



2. Proof objects may be ranged under three heads: 

 First, those which render manifest chromatic and spherical 



