450 On Sun Pictures. 
for instance—the maximum strength of the heating ray is 
found to lie between the colours red and crimson ; that of 
the lighting ray between the yellow and orange colours ; 
whilst the greatest power of the chemical ray is between 
the violet and indigo. Curious as it may appear, it is 
nevertheless true, that neither the light which we see, nor 
the heat which we feel, have any thing to do with the 
production of pictures by the sun; it is to the actinic, or 
chemical and invisible ray, that we are indebted for the 
wonderful power of so minutely delineating Nature's works 
and copying the master-pieces of art. 
The photographist can easily prove the above fants to 
be correct by the following experiments :—Let him prepare 
apiece of photographic paper in the usual most simple 
manner, namely, by a wash of ammonio-nitrate of silver, 
This paper, if exposed to the sunlight in Hobart Town, 
at this season of the year, ought to become quite black in 
less than ten minutes. By preparing a second piece of paper 
in the same manner, and submitting it to the sun’s rays, with 
a piece of yellow glass placed over it, the paper will be found 
nearly insusceptible of change; but if the same prepared 
paper be similarly exposed, with a piece of blue glass, of a 
‘colour so deep as apparently to obstruct all light, placed over 
it, the paper will blacken nearly as fast as if it were ex- 
posed to the unveiled sunlight. The knowledge of this fact 
enables me to prepare the papers (in that part of the process 
which would suffer from daylight) without the aid of a 
lighted candle, by the substitution of a medium of yellow 
calico, which thus cuts off the actinic rays, while it allows 
those of light to pass through freely. The following experi- 
ment shows conclusively the different degrees of actinic power 
which belong to the various portions of the spectrum. ‘The 
rays being thrown by means of the prism upon a sheet of 
