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. 
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A 7 
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