



IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 787 
PRACTICAL APPLICATION IN SPECTROSCOPY AND OPTICS. 
One of the most frequent utilisations of coloured glasses in solar spectro- 
scopy is, to keep out of the field of view false glare from neighbouring and 
brighter parts of the spectrum. Wherefore arises the useful question,—What 
colour shall we adopt to transmit the utmost possible quantity, say, of the faint 
red end, and extinguish the utmost of every other part, of the spectrum ? 
Then comes the answer, strange evidently to the authors of some of the 
text-books, though well known to experienced observers,—“ Use either blue or 
green, if dichroic, media; for they transmit red light, more red, or further to 
the red end of the spectrum, than any known red medium whatever.” But the 
said blue or green media must be very intensely dichroic, and all the stained 
green glasses I have yet tried, have been exceedingly mono-chroic and there- 
fore useless, even pernicious, in such an inquiry. 
The quality demanded, or for transmitting red light, comes out numerically 
quantified from our observations, thus,— 
OF THE REDNESS OF RED LIGHTS, 
by its Wave-number Place in the Spectrum. 









Number of 
Waves at the Class of Colour. 
Intensity of place ina 
Colouring Material, and its depth. pee ee L : Been anol: 
Max. = 10. decreases as | Mono- Di- Poly- 
the number | chroic. | chroic. | chroic. 
increases. 
Two Cobalt-blue glasses, 4:1 35 587° x 
zoo Of Cambridge Blue dye, ag 36 166° x 
109 of Judson’s Green dye, . 3 36 765 x 
gs Oxalate of Chromium and Potash; ‘Blue, 4 36 928 ae x 
Two Blue-lilac glasses, Eis) Biol ws ee x 
Claret wine ‘Purple-red, by day-light inispection, a" 37 411 os 
sid Litmus Blue, or Or 779 alate x 
Lisbon Collares wine, . 3 37 879 x 
100 Judson’s Violet dye, a 38 023 : x 
ato Judson’s Mauve-red, 3 38 226 ae x 
sto 5 Judson’s Purple, 3 38 285 ooh x 
zi Judson’s Peach-blossom, 4 38 343 = x 
Leith supplied Port wine, 3 38 462 x 
1, Marone-red dye, 4 38 820 x 
Four Ruby-red glasses, 4°5 39 371 x 
abo Judson’s Crimson, 4 39 417 x 
1, Judson’s Ruby dye, + 39 526 ag x 
x b ¢ Lodine solution, Brown in simple day-light, 4 39 526 6 
abo Magenta Red, : 4 40 016 ear x 
vat Permanganate of Potash, Pinkish, 4 40 016 x 
Lisbon Port wine, Brown-red, 4 40 650 ae x 
Two Red-lilace classes, . 4°5 40 933 oe x 



VOL. XXVIII. PART III. 9U 
