HERSCHEL. 209 



under the same degree of magnifying power as the direct light of the 

 fixed stars does. 



Opticians had given up, more from theory than from careful experi- 

 ments, attempting high magnifying powers, even for reflecting telescopes. 

 They thought that the image of a small circle cannot be distinct, cannot 

 be sharp at the edges, unless the pencil of rays coming from the object 

 in nearly i^arallel lines, and which enters the eye after having passed 

 through the eye-piece, be sufficiently 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 

 conditions, grafted on each other, vanished in presence of Herschel's 

 observations. After having put himself on his guard against the eflects 

 of difiraction — that is to say, against the scatternig that light undergoes 

 when it passes the terminal angles of bodies — the illustrious astronomer 

 proved, in 17SG, that objects can be seen well defined by means of pen- 

 cils of light whose diameter does not equal five-tenths of a milliaieter. 



Herschel considered the almost unanimous opinion of the double-lens 

 eye-piece being preferable to the single-leus eye-piece as a very injurious 

 prejudice to science, since experience proved to him, notwithstanding all 

 theoretic 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 v/ith double 

 eye-pieces. On one occasion this latter eye-piece could not show him 

 the bands of Saturn, while by the aid of a single lens they were perfecitiy 

 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 vis- 

 ion." (Philosophical Transactions, 1782, i>p. 91 and 95.) 



It is not only relative to the comparative merit of single or double 

 eye-pieces that Herschel diflers in opinion from opticians generally. 

 He thinks, moreover, that he has proved, by decisive experiments, that 

 concave eye-pieces (like that used by Galileo) surpass the convex eye- 

 piece, both as regards clearness and definition. 



Herschel assigns the date of 177G to the experiments which he made 

 to decide this question. (Philosophical Transactions, year 1815, p. 297.) 

 Plano-concave and double-concave lenses produced similar eflects. In 

 what did these lenses difler from the double-convex lenses? In one 

 particular only: the latter received the rays reflected by the large mir- 

 ror of the telescope after their union at the focus, whereas the concave 

 lenses received the same rays before that union. When the observer 

 made use of a convex lens, the rays that went to the back of the eye to 

 form an image on the retina had previously crossed each other in the air, 

 but no crossing of this kind took place when the observer used a con- 

 cave lens. Holding the double advantage of this latter sort of lens 

 over the other as quite proved, one would be inclined, like Herschel, 

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