36 Frauenhofer on the R^r active and IHspersive Power 
observations of this kind, no other light but that of the spec- 
trum ought to enter into the field of the telescope, and that 
the wire should not receive any foreign light. I have found, 
with a lens of crown-glass. No. 13., with a focus of 0.88 Paris 
inches, that the eye-glass, in passing the wire from the ray C to 
G, ought to be displaced 0.054 of an inch, in order to see the 
wire with equal distinctness in both colours. A lens of crown- 
glass, No. 13., of 1.33 Paris inches focus, requires to be displaced 
0.111 for the same colours ; a lens of flint glass, No. 30., with a 
focus of 0.867, requires a displacement of 0.074 ; and another 
of flint-glass. No. 30., and a focus of 1.338, required to be dis- 
placed 0.148 of an inch. In these experiments, I looked with 
one eye at a fixed object, whilst with the other I observed at the 
wire through the lens, in order that I might be certain that, 
with the different coloured rays, the eye was always equally sus- 
ceptible of uniting on the retina white rays of a given divergency ; 
and, consequently, that it did not change, in that respect, for the 
different colours. Even with that precaution, however, the re- 
sults did not differ greatly from the preceding. 
The result given by the first lens is, that if the red rays 
fall parallel on the eye, the blue rays ought to diverge from a 
point 23.7 inches distant, in order to have in the eye the same 
focal distance. With the second lens this distance was 21.3; 
with the third 19.5; and with the fourth 17.9. In this calcu- 
lation, I have taken into account the influence produced on this 
displacement by the unequal refrangibility of the two kinds of 
rays in the lens. This aberration in the eye cannot be fixed 
more rigorously, but by varied and repeated trials. It would be 
desirable to have the experiments repeated on the eyes of diffe- 
rent persons, in order to obtain a mean result. In order to de- 
termine this aberration with still more precision, we must also 
take into account the diameter of the luminous cylinder formed 
where there is a sudden change of density ; and Dr Young has remarked, that if 
this hypothesis is well founded, the dispersion of the eye must be attributed wholly 
to the aqueous humour. Mr Ramsden’s opinion is subversive of every principle in 
optics, and does not deserve to be refuted. Dr Brewster found that the disper- 
sion of his eye, at the margin, when the pupil w^as about the seventh of an inch in 
diameter, was corrected by a prism of flint-glass of 10°, and having a dispersive 
power of 0.0478 .— Tka NS, 
