1875.] The Mechanical Action of Light. 349 
This form of instrument is of too recent a construction for 
me to beable to do more than draw brief attention toa few of 
the many uses for which it is applicable. 
By timing the revolutions of the instrument when exposed 
direct to a source of light—a candle, for instance—the total 
radiation is measured. If a screen of alum is now inter- 
posed, the influence of heat is almost entirely cut off, the 
velocity becomes proportionately less, and the instrument 
becomes a photometer. By its means photometry becomes 
much simplified ; flames the most diverse may readily be 
compared between themselves or with other sources of light ; 
a “standard candle” can now be defined as one which 
at x inches off causes the radiometer to perform y revo- 
lutions per minute, the values of x and y having pre- 
viously been determined by comparison with some ascertained 
standard; and the statement that a gas-light is equal to 
sO many candles may, with more accuracy, be replaced 
by saying that it produces so many revolutions. 
To photographers the radiometer will be invaluable. As 
it will revolve behind the orange-coloured glass used for 
admitting light into the so-called dark room, it is only 
necessary to place one of these instruments in the window 
to enable the operator to see whether the light entering his 
room is likely to injure the sensitive surfaces there exposed ; 
thus, having ascertained by experience that his plates are 
fogged, or his paper injured, when the revolutions exceed, 
say, ten a minute, he will take care to draw down an extra 
blind when the revolutions approach that number. Still 
more useful will the radiometer be in the photographic 
gallery. Placing an instrument near the sitter at the com- 
mencement of the day’s operations, it is found that, to obtain 
a good negative, the lens must be uncovered—not for a par- 
ticular number of seconds—but during the time required for 
the radiometer to make, say, twenty revolutions. For the 
remainder of the day, therefore, assuming his chemicals not 
to vary, the operator need not trouble himself about the 
variation of light ; all he has to do is to watch the radiometer 
and expose for twenty revolutions, and his negatives will be 
of the same quality,* although at one time it may have taken 
five minutes, and at another not ten seconds, to perform the 
allotted number. 
‘ * Tn this brief sketch I omit reference to the occasions in which the ultra 
violet rays diminish in a greater proportion than the other rays. 
