Grusps—WNotes on a Paper by Professor E. Hale. 525 
Referring to the author’s Paper of 1877, he corroborates what 
is there said about the distinctive advantages of each form, and 
then enters into a most exhaustive and valuable examination of the 
relative light collecting powers of refractors and reflectors when 
used for visual work and when used for photographic work. 
An elaborate table is given of the various results of his analyses, 
and also a diagram which represents the same in a graphical form, 
which latter is here reproduced (see p. 526). 
An examination of this diagram will show that whereas the 
refractor is a more powerful light collector for visual rays 
up to about four feet in diameter, the reflector then, and for sizes 
over this aperture, becomes the more powerful in consequence of the 
fact that owing to the increased thickness of the glass the absorption 
of light is greater. The difference, however, is never very great 
for visual rays. In the case of photographic rays, however, the 
absorption in the case of the refractor is so great that at one metre 
diameter the reflector becomes 50 per cent. more powerful; at 2 
metres it is 100 per cent. more powerful than a refractor of equal 
size; while at a little over 3 metres aperture the absorption 
increases at such a rate that no more light is collected by increasing 
the aperture. 
The conclusions, therefore, which Professor Hale comes to are 
as follows. He says: “ As regards the future development of 
telescopes in the direction of increased light-grasping power, the 
reflector promises far greater gains than the refractor, especially 
for spectroscopic work in the so-called photographic region. Indeed 
it appears from an inspection of the curves in the diagram that an 
increase in the aperture of an objective beyond about 350 centi- 
metres would be attended by no gain in the intensity of the photo- 
graphic image. In the case of reflectors, on the contrary, the 
light grasping power will continue indefinitely to gain with the 
aperture.” 
The author desires to take this opportunity of thanking Profes- 
sor Hale for his courteous reference to the Paper above referred to 
in the Transactions of the Royal Dublin Society. 
SCIEN. PROC. R.D.S., VOL. VIII., PART V. 2Q 
