842 PROFESSOR PIAZZI SMYTH ON COLOUR, 
5. DIDYMIUM NITRATE—continued. 

Solution=+ strength—continued. 








Intensity. | Intensity. W. % ber || Golour erie 
Light | Dark | 7 | in Bruit. inch. | Wave-number, 
Extremely faint lime . : : i 1 26°46 47 710 | GREEN 
Blacker (1st edge . : ne see 26°52 48 309 | GREEN 
Sein ates ite NeiGletc teste. ||) me 8 2658 | 48 828 | GREEN 
. Ist edge . B. oe 26°65 49 407 | GREEN 
Wonaig) cs sea Tee {oa edge .| ... | 2 | 2671 | 49 925 | Green 
Maximum BuveE Licut . : : 4 aap 26°82 50 916 | GLAUCOUS 
Dark hazy line : 5 27°04 52 910 | Guaucous 
Faint hazy line. , 2 2711 53 476 | GLaucous 
Broader and darker line : ae 6 27-21 54 201 | GLaucous 
Maximum Inpico Licut : ; 3 ee 27-42 55 741 | BLUE 
1st edge . af a 27°54 56 657 | BLUE 
eae | Qdedge.| g | 2768 | 57 703 | Inpico 
Maximum Vio.er Licut : , 2 oe 27:81 58 548 | VIOLET 
Faint hazy line. : : : ee -. 27°96 59 453 | VIOLET 
End all ' : : : : 0 see 28°21 61 087 | LAVENDER 


PLATE I., No. 41 oF rats Vou. XXVIII. 
This is a representation in black and white, of the effects of three degrees or depths of colour, of each 
of three diverse kinds of stained glass, viz., ruby-red, rich green, and cobalt-blue, in stopping out certain 
parts of the spectrum of a bright coal-gas flame. 
The method of representation is symbolical, not imitation. In the latter case the spectrum-strip 
would have been of the same height from end to end ; while bright bands and dark bands, all of equal 
height, would have differed from each other, solely in most refined degrees of shade; an ultra difficult 
pictorial result to produce and keep to, when the strictest economy in engraving is enforced on an author. 
The symbolical method has been therefore adopted, wherein degrees of light or shade are represented by 
amount of height above, or depth below, a certain horizontal base-line ;—a method both easy to realise 
in any kind of multiplication of graphical work, and easy also of recognition by the student. 
In either case, the spectrum necessarily begins and ends in darkness. The locomotion of colour- 
bands, with increased depth of the transparent colouring material (so much alluded to in the paper) is 
shown by the strong dotted lines connecting several spectra together. (See also p. 781.) 
PLATE IL, No. 42 or rais Von. XXVIII. 
Here, for economy’s sake, only two of the many fluids examined have been dealt with. One of 
them, Judson’s green, differs from the stained-green glass of Plate L, by being, though equally green, 
not a mono-chroic, but a di-chrote ; and the parent on that account of many remarkable phenomena 

