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van. 12, 1882 
see it does not come to nearly such a sharp point as when a 
thermo-pile is used, and you ask why. The reason is that at 
that particular part the bromide of silver—blue bromide—does 
not entirely absorb the radiations, but allows a certain amount to 
pass through. Nevertheless you will see that there is a striking 
similarity between the two. 
Now I wish to show you how you may combine the thermo- 
graphs of two or more temperatures. I must, first of all, show 
you the thermographs of varying temperatures (Fig. 23). 
have a temperature of 250°. 
500° ; next 2,000, next 4,000, 
In diagram (Fig. 24) we have a combination between the 
thermograms of two temperatures, one of about 2,000°, and the 
other of about 4,000°. By measuring the height of these curves 
We 
Fig. 24.—Combination thermogrem. 
and taking the mean you get the central curve. You should 
compare this with Prof. Tyndall’s curve. 
very different from the curve taken from the combination of the 
two curves. My time, however, is drawing to a close, and I 
am obliged to go but shortly through this part. 
In my last lecture I showed you how the diffraction spectrum 
was spread out in the infra-red in comparison with the prismatic 
spectrum, and I think that it may interest you here to show you 
the way in which a thermogram of the solar spectrum is spread | 
out when the prismatic thermograph is altered into a refraction 
thermograph or thermogram. 
In Fig. 26 I givea diagram of a solar prismatic spectral ther- 
mogram (Fig. 25) as obtained by Lamarsky spread out into a 
diffraction curve. You see that instead of the maximum heating 
effect of the solar spectrum being beyond the red, it Jies well 
between E and D. In other words the maximum energy of the 
solar spectrum lies in the yellow and not in the ultra red as has 
usually been considered. 
The energy of a wave, or a series of waves, is measured by 
the square of the amplitude divided by the square of the wave 
length into a constant. The area of wave section is equal to 
the amplitude—that is to say the height of the wave multiplied 
by the length of the wave into a constant. If these waves have 
equal sectional areas, the energy varies inversely as the fourth 
power of the wave length. And what I wish to draw your 
attention to is this—that starting from the theoretical limit of 
the prismatic spectrum to the maximum heating effect of any 
continuous spectrum a law seems to hold that the energy of any 
portion of the spectrum below its point of maximum energy does 
vary inversely as the fourth power of the wave length. 
I am sorry I haye not time to go farther into the detail of this, 
Next we have a temperature of | 
but it has been the result of some considerable calculation, and 
experiment. 
After my last lecture I was asked whether the photograph 
taken by the kettle in the manner explained was not due to the 
heat rays. Iam afraid my reply was somewhat short as I said, 
‘ There are no such things as heat rays.” I think that now may 
be an opportunity in which to express my views on the subjeet 
ina less curt manner than that in which I answered my ques- 
tioner. It is true that we often do hear of dark heat rays and 
of radiant heat, and the rays which are principally concerned in 
| the latter definition are taken to lie in the infra-red region of the 
| spectrum. I would ask, ‘‘ Why give them a name to which, it 
| seems to me, exception can be justly taken?” In 1800, Sir 
| William Herschel proved that these dark rays could be refracted 
} 
Fic. 25.— Prismatic thermogram of solar spectrum obtained by Lamansky. 
; and reflected like those rays which, falling on our retina, give us 
the sensation of colour. Professor Forbes, in his celebrated 
| experiments, proved the same thing ; but, in addition, he like- 
wise proved that they could be polarised. I think I have laid 
before you proofs that these same rays can expend their energy 
in chemical action causing a disruption of a molecule by their 
successive impacts. Those rays, by whose agency we see, exer- 
cise the same functions as these dark rays.” All rays are alike, 
and whether they causea rise in temperature, or cause a chemical 
decomposition of a body, depends solely on the nature of that 
body on which they fall. The waves, as I have tried to demon- 
strate, carry energy and nothing else; and they must meet with 
some obstruction which shall destroy their motion before they 
You see it is not, 
can show that they possess energy. The work done by them is 
Fic. 26.—Difiraction thermogram from Fig. 25. 
manifested by either molecular motion, or atomic motion, or both; 
the molecular motion of the body showing itself perhaps as heat, 
and the atomic motion as chemical action. If we must have the 
word ‘‘radiant” tacked on to a definition, (and the word 
radiant” is a remnant of the corpuscular theory of heat,) alt 
the wave motion in the ether should be classed under the head of 
‘*radiant energy.” If a shorter nomenclature is required, let us 
simply call it ‘‘ light,” including in it the energy carried. Light 
is an old word understood in one sense by all, and we need only 
talk of the heating effect of light, and so on. The word 
“‘actinism ” falls into an equal condemnation, We have un- 
luckily none of our most eminent philosophers who are scientific 
photographists, if there were I do not believe any would defend 
the retention of the word ‘‘actinic” ‘or chemical rays” amongst 
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