338 
REPORT —1854 
When by artificial means the luminiferous rays are analysed, some rays 
are found in which great heating power coexists with feeble illuminating 
power, but under the same conditions and for the same ray, the heating is 
probably directly proportional to the illuminating intensity. 
In this way it was that from the distinctions obtained in my original expe¬ 
riments (1825), I was led to describe generally the communication of heatai 
effected in three distinct ways:— 
(I.) By conduction (including what some term convection). 
(2.) By radiation (in the ordinary sense of the term), or mere cooling ot I 
hot body (Species I. above). . 
(3.) By the agency of light, whether fYom the sun or flames (Species 11. 
and III.). 
In the first instance, it was natural to regard these as so many distinct 
kinds of physical action producing heat, but more recent researches, espe¬ 
cially those of Professor Forbes, have enabled us to conuect them hr Hie 
simple and uniform analogy supplied by the undulatory hypothesis, It* 1 - 
adopt the hypothesis of uudulatiuns of decreasing lengths, those oMIw 
greatest length correspond to the Species I., these continue to be 
out as the temperature is raised, or as combustion proceeds more intensely- 
along with others of successively less wave-length, until we arrive at 
cies II., or those which are of the proper length to affect also the retina w» 
the sense of vision ; and at lust the wave-lengths arc too small to pnwJ'ice 
either luminiferous or calorific effects; but here they seem to obtain we 
maximum of chemical action. But rays of all wave-lengths thus continue 
to be given out simultaneously. They all produce or excite heat mwf ^ 
less when stopped or absorbed; and this probably dependent, in some 
ratio, on the greater wavo-length simply; while the illuminating efiectdeptut^ 
on some peculiar relation to certain wave-lengths only determined b\ sj>i' 
physiological conditions of the retina at present unknown. Substances^ ^ 
are transparent transmit freely rays of the visual wave-lengths, w 
course carry with them their heating powers. „ 
Opake substance!* which are diathermanous transmit in the same"’ 3 ) 
of the longer wave-lengths, but not those of shorter. , 
The action of the texture of surfaces seems purely median 0 n 
bably influences the absorption of all rays, but its full effect is pronuc ^ 
the rays ol longer wave-length; while that of colour is purely > 
applies to rays of luminiferous wave-lengths only. ^ 
Refraction, polarization, interference, and the like properties ol % _• , 
would be easily' seen, must be accompanied with just such indications “ 
heating effect as might consist with the modification which the /«# in 
respective cases, might undergo. If the light were extinguished 
conditions nf i mint !sutler, .. .1 _ 1 . .M n( miuiscW ®1 
conditions of polarization or of interference, the heat would of course 
pear with it, and the changes of intensity' would be similar; that is, ** 
portion ol the rays which consists of Species II. ' .. 
Ilnis, in a multitude of such cases, these effects might be very, am 
dlS?UI /n r i ‘ hi(kl ‘ J,K And, moreover, if they were real . j 
mWe for Species (II.), U M ould not follow that they existed for Spec*® ^ 
yet the wave analogy would render it highly probable; and expend “ 
ratio" of therise of s , lnn,lest difference in the nbsolnte values,and none 
and'often verv great In n*' ° I» the brightest terrestrial rays there W* 
coneluionsaftothl ^ ,£ f ertnce as well as amount; this constituted the ground^ 
conclusions as to the heterogeneity of the species of rays in the latter case. 
