compound lenses and object-glasses. 249 
position, for the lens next the eye, and a plano-concave whose 
focal length is to that of the other as 2 6 : 1 or as 13:5, 
placed in contact with its flatter surface and having its con- 
cavity towards the object, as in PI. XIX. fig. 6, for the farthest : 
yet for destroying the aberration of rays parallel to the axis, 
nothing can be worse. In fact our formula (/>) gives for the 
aberration in this construction 
— = 22*302 
or about 22 times what the best single lens of equal power 
would give : yet on accidentally combining two such lenses in 
this manner, I was immediately struck with the remarkable 
extent of oblique vision,* with the absence of fatigue, on read- 
ing some time with a power much beyond that of the natural 
eye, and with the freedom from colour at the edges of the field, 
arising from the opposition of the prismatic refractions of the two 
solids, an advantage which a single meniscus does not possess. 
Theory of object-glasses ; and first, of the destruction of the chro- 
matic aberration , or the imperfections arising from the different 
refrangibility of the rays of light. 
15 The perfection of an object-glass requires that parallel 
or diverging rays of all colours incident on every point of the 
glass, should converge to one and the same point, and con- 
sequently, that we should have/ invariable and A/ zero, for 
all the colours of the spectrum. With regard to the latter 
* The focal length of the compound lens tried, was 1*84 inch ; the field of tole- 
rably distinct vision extended full 40° from the axis, and the forms of objects were 
distinguishable (the letters of a book might be read) with management, as far as the 
75th degree. The lenses used in this combination should be very thin, and the eye 
applied as close as possible. 
