IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 815 
COMBINATIONS OF COLOURED GLASSES—continued. 
But— 
(1’.) 2 Rep + 2 GREEN, give a rather narrow band of brown-orange, useful for definition 
and pleasant to the eye as a sun-shade glass. 
(2'.) 2 YetLow-Brown + 2 GREEN give a rather broad band not good for sun-shade or for 
correction of non-achromaticity of image, but good enough for ordinary vision; and as its 
centre is nearly on the D line, such illumination is pleasanter to the eye, and more like white 
light to it, than the yellow-brown alone, when used for anti-actinic purposes in ordinary 
muriatic, iodic and possibly some bromo-iodic, photography. 
(3’.) 2 BLUE + 2 Rep give a rather broad band over the place of the Solar B line. 
This combination affords a very safe, though not an agreeable, light for a photographer’s 
dark room, when working with the most sensitive kinds of pure bromide of silver preparations. 
It is necessary too ; for the Bromide, as Sir John Herschel showed nearly forty years ago, 
extends its action over “an extravagant length” of the Solar spectrum, as compared with the 
very limited range through which Iodide of silver is sensitive. 
And— 
(4’.) 4 Buur + 1 Rep, where the 1 red cuts off the blue light of the blue, and does not 
much interfere with the ultra red of the blue, though dragging it somewhat back from the 
ultra red direction, as from A to a, Solar, gives a narrower band of peculiarly rich deep red. 
This band, as to intensity of light, is weak; and it is far removed from the ordinary 
visual regiong of the spectrum, but it must possess peculiar monochromatic, and for most 
chemicals anti-actinic, power; unless indeed the light be very strong, when there is always 
danger of the green and blue lights coming in also. 
