with Application to Photometry. 447 
upon the prism or diaphragm upon which the eye is focused. 
If desired, an eye-lens may be placed at the second slit ; 
but this is not generally needed. 
In the present instrument a direct-vision dispersing prism 
is used, so that the optical parts can be all disposed in a 
narrow box of nearly 3 feet in length. ‘The lenses are all 
single lenses, and work sufficiently well. The slits are of such 
width that either coincides with the image of the other, and 
their relative position is so chosen that the mean refrangibility 
of the light is that corresponding to sod‘um. Objects seen 
through the instrument thus appear as if lighted by a sodium 
flame. 
The principal object which I had in view in the construc- 
tion of the instrument now exhibited was to see whether it 
could be made of service in the comparison of compound 
lights of somewhat different colours—a problem just now 
attracting attention in connection with electric lighting. It 
is scarcely necessary to say that a comparison of this kind is 
physically incomplete unless it extends to all the spectral 
components separately ; but for commercial purposes such an 
extended comparison is too complicated, and indeed useless. 
Determinations at two points of the spectrum, as proposed 
by Capt. Abney, would certainly suffice for ordinary purposes; 
and in view of the convenience of expressing the result by a 
single number, it is not unlikely that people practically con- 
cerned in these matters will content themselves with a com- 
parison at one point. It seems desirable that some convention 
should be arrived at without much further delay; so that two 
lights should be considered to be commercially equal, if they 
have the same intensity of, e. g., sodium or of thallium light. 
It will be understood that such a mode of estimation assumes 
that the intensity varies along the spectrum in a gradual 
manner ; and this consideration may tell against the use of 
the sodium light as a standard, inasmuch as the component in 
question often predominates unduly in candle-flames from the ~ 
actual presence of sodium. 
Whatever choice be made, an instrument like the present 
may be employed to make the desired selection ; and it is 
applicable to any photometric arrangement. For my own 
experiments I have used the shadow-method, and find it pos- 
sible to compare any bright sources, however different in colour. 
The only difficulty arises from the necessary enfeeblement of 
the light by selection, and this practically precludes observa- 
tion with standard candles. With gas-flames and glow-lamps 
the light is sufficient. — 
2H 2 
