446 Lord Rayleigh on a Monochromatic Telescope, 
or in terms of m, n, p, - 
Piss A aa. 
2m+n)p+1—p 
When mand n are great, both expressions reduce to n/(m-+-n) ; 
so that the light passing in the two directions is equally bright. 
Moreover, and this is the point to be especially noticed, how- 
ever great m may be—thatis, however impervious the first pile 
is, the light between the two piles may be made to approach 
the original light in brightness as nearly as we please, by suffi- 
ciently increasing the number of plates in the second pile ; 
that is, the light between the piles may be made to be the same 
as if the first pile were removed. [rom this example we may 
understand more clearly how a very small quantity of light 
penetrating directly may be beaten backwards and forwards, 
as between two reflectors, until the original intensity is 
recovered. 
XLIX. A Monochromatic Telescope, with Application to 
Photometry. By Lord Rayueteu, #.R.S.* 
ee purpose of this instrument is to exhibit external objects 
as they would be seen either with the naked eye, or 
through a telescope, if lighted with approximately monochro- 
matic light ; that is, to do more perfectly what is done roughly 
by a coloured glass. 
The arrangement is not new, though I am not aware that 
it has ever been described. In 1870 I employed it for deter- 
minations of absorption, and, if my memory serves me right, 
I heard soon afterwards from Clerk-Maxwell that he also had 
used it. It is, indeed, avery slight modification of Maxwell’s 
colour-box. 
In the ordinary form of that instrument, white light ad- 
mitted through a slit is rendered parallel by a collimating 
lens, dispersed by flint-glass prisms, and then brought to a 
focus at a screen, upon which accordingly a pure spectrum is 
formed. This screen is perforated by a second slit, immedi- 
ately behind which the observer places his eye. It is evident 
that the light passing the aperture is approximately mono- 
chromatic, so that the observer, if he focuses his eye suitably, 
will see the prism illuminated with this kind of light. The 
only addition now required to convert the instrument into a 
monochromatic telescope is a lens placed just within the first 
slit, of such power as to throw an image of external objects 
* Communicated by the Physical-Society: read April 25, 1885, 
