5 ° 
Dr. Young's Lecture 
rays; and the eye must also have been encumbered with a mass 
of much greater density than is now required, even for the 
central parts : and, if the whole lens had been smaller, it would 
also have admitted too little light. It is possible too, that Mr. 
Ramsden's observation,* on the advantage of having no re- 
flecting surface, may be well-founded : but it has not been de- 
monstrated, that less light is lost in passing through a medium 
of variable density, than in a sudden transition from one part of 
that medium to another ; nor are we yet sufficiently acquainted 
with the cause of this reflection, to be enabled to reason satisfac- 
torily on the subject. But, neither this gradation, nor any other 
provision, has the effect of rendering the eye perfectly achro- 
matic. Dr. Jurin had remarked this, long ago,^ from observing 
the colour bordering the image of an object seen indistinctly. 
Dr. Wollaston pointed out to me on the optometer, the red 
and blue appearance of the opposite internal angles of the cross- 
ing lines; and mentioned, at the same time, a very elegant ex- 
periment for proving the dispersive power of tire oye. He looks 
through a prism at a small lucid point, which of course becomes 
a linear spectrum. But the eye cannot so adapt itself as to make 
the whole spectrum appear a line; for, if the focus be adapted 
to collect the red rays to a point, the blue will be too much re- 
fracted, and expand into a surface ; and the reverse will happen 
if the eye be adapted to the blue rays ; so that, in either case, the 
line will be seen as a triangular space. The observation is con- 
firmed, by placing a small concave speculum in different parts 
of a prismatic spectrum, and ascertaining the utmost distances 
at which the eye can collect the rays of different colours to a 
focus. By these means I find, that the red rays, from a point at 
* Phil. Trans, for 1795, p. 2. f Smith, e. 96. 
