HIS IMPROVED EYE-PIECES. 279 
sharp at the edges, unless the pencil of rays coming from 
the object in nearly parallel lines, and which enters the 
eye after having passed through the eye-piece, be suf- 
ficiently broad. This being once granted, the inference 
followed, that an image ceases to be well defined, when it 
does not strike at least two of the nervous filaments of the 
retina with which that organ is supposed to be overspread. 
These gratuitous circumstances, grafted. on each other, 
vanished in presence of Herschel’s observations. After 
having put himself on his guard against the effects of 
diffraction, that is to say, against the scattering that light 
undergoes when it passes the terminal angles of bodies, 
the illustrious astronomer proved, in 1786, that objects 
can be seen well defined by means of pencils of light 
whose diameter does not equal five tenths of a milli- 
metre. 
Herschel looked on the almost unanimous opinion of 
the double lens eye-piece being preferable to the single 
lens eye-piece, as a very injurious prejudice in science. 
For experience proved to him, notwithstanding all the- 
oretic deductions, that with equal magnifying powers, in 
reflecting telescopes at least (and this restriction is of 
some consequence), the images were brighter and better 
defined with single than with double eye-pieces. On one 
occasion, this latter eye-piece would not show him the 
bands of Saturn, whilst by the aid of a single lens they 
were perfectly visible. Herschel said: “'The double eye- 
piece must be left to amateurs and to those who, for 
some particular object, require a large field of vision.” 
(Philosophical Transactions, 1782, pages 94 and 95.) 
It is not only relative to the comparative merit of 
single or double eye-pieces that Herschel differs from 
the general opinions of opticians; he thinks, moreover, 
