On Sun Pictures. 449 
Before leaving this part of the subject I must mention, 
as a curious fact, that these pictures cannot be taken quite 
so quickly here under the glorious sunshine, which go daz- 
zles our eyes, as in the apparently unfavourable atmosphere 
of London,—I speak of the west end of London, not of the 
city, and of course exclude the real London fog days. 
The reason is, that the quality of light here ig too yellow 
for photographic purposes. ‘he more northerly we go, the 
whiter the light becomes ; and the whiter the light, the quicker 
a photographic picture of any kind can be taken. 
Tt is well known that every beam of the sun’s light is 
composed of a collection of rays, which may easily be sepa- 
rated and shown apart by allowing the beam of light to pass 
through a common glass prism, by which they are refracted, 
and may be thrown obliquely upon any white surface. 
This spectrum (as it is called) will then be found to con- 
sist of nine rays (formerly believed to be only seven), of the 
following colours, and placed in the following order :— 
erotinitaripacviun, Lavender 
Qian ccs aitvebscstenttiotede att Violet 
OY arbour Indigo 
7 EF Rrcrer Perret w»» Blue 
OPE sees eirsacrt Green 
Ge isp atipcce soprsast ative ine Yellow 
(O auustospvarenpam Orange 
Oo dp natatinadintie Red 
OF srr i.liercibte Crimson. 
It is likewise known that each collective beam or ray 
which proceeds from the sun possesses three distinct proper- 
ties—namely, the property of heat, of light, and of actinism, 
or chemical power. 
By experiment we can determine to which portion of the 
coloured spectrum each of these three Properties belongs ; 
UU 
