280 HERSCHEL. 



that he has proved by decisive experiments, that concave 

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

 eye-piece by a great deal, both as regards clearness and 

 definition. 



Herschel assigns the date of 1776 to the experiments 

 which he made to decide this question. (Philosophical 

 Transactions, year 1815, p. 297.) Plano-concave and 

 double concave lenses produced similar effects. In what 

 did these lenses differ from the double convex lenses ? 

 In one particular only : the latter received the rays re- 

 flected by the large mirror 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 

 crossed each other before in the air ; but no crossing of 

 this kind took place when the observer used a concave 

 lens. Holding the double advantage of this latter sort 

 of lens over the other, as quite proved, one would be in- 

 clined, like Herschel, to admit, " that a certain mechanical 

 effect, injurious to clearness and definition, would accom- ' 

 pany the focal crossing of the rays of light." * 



This idea of the crossing of the rays suggested an ex- 

 periment to the ingenious astronomer, the result of which 

 deserves to be recorded. 



A telescope of ten English feet was directed towards 

 an advertisement covered with very small printing, and 

 placed at a sufficient distance. The convex lens of the 



* On comparing the Cassegrain telescopes with a small convex 

 mirror, to the Gregorian telescopes with a small concave mirror, Cap- 

 tain Kater found that the former, in which the luminous rays do not 

 cross each other before falling on the small mirror, possess, as to in- 

 tensity, a marked advantage over the latter, in which this crossing 

 takes place. 



