OF RADIANT HEAT THROUGH DIFFERENT BODIES. 43 
_~ A-similar construction will give the curves a!/b"cld!'ellflg!', all! Bl! 
cl diltell' f'l' g!"', alV 61V,. representing the decreasing intensities, of the 
three.other radiations... .. ; “5 c Sent meee 
Let.us.now suppose the screen cut by any plane (P P’) parallel to ON; 
the emergent rays of the detached plate will be determined by the points 
at which the plane intersects the curves; so that PP’, PP", PP,!”’ will re- 
present the quantities of heat that issue from the plate OP when.exposed 
to the first three sources; for the rays of the fourth are completely ex- 
tinguished at the distance of one millimetre. We now see that the ra- 
tios of the distance from those points of intersection to the axis.QOM 
decrease. in proportion as the.thickness of the interposed layer is less. 
The distances from those points to the axis are pretty nearly equal when 
the section. coincides with the ordinate aa! at: which the observations 
commence; they will become yet more so in the interior of the first layer 
O a, so that within a limit: very close to the surface.at which the rays 
enter the differences will almost.vanish*. to bai 
The first infinitely thin plate will therefore transmit sensibly equal 
quantities of radiant. heat from the four sources. The diminutions how- 
eyer which the rays from each source. will suffer.in the interior. of .this 
elementary plate, though so exceedingly small that they may be disre- 
garded in reference to the quantities. transmitted, must. nevertheless 
bear very different ratios to one another ; for it is to such diminutions, 
several times repeated by the action of the successive. layers, that we 
are to attribute the remarkable differences in the .quantities.of heat 
transmitted from each source by a screen of a given thickness. 
The law of Delaroche did not show whether the variable interception 
* T have been unable to procure plates of glass thinner than +7; of a milli- 
metre. But we shall see presently that all other diaphanous substances, whether 
natural or artificial, are in their effects more or less analogous to glass. Now 
there are several crystals which spontaneously separate into plates of great te- 
nuity, and are, consequently, well calculated to show that the ratios of the quan- 
tities of heat transmitted by a screen exposed to the radiations of the four sources 
approximate to equality in proportion as the thickness of the screen is reduced. 
Thus a plate of sulphate of lime 2™™-6 in thickness gave for the four transmis- 
sions respectively, 
Te 5 tO, OF 
These transmissions became 
3G0.18,).: 7660 
when the thickness was reduced to 0™™-4; and 
64, 51, 32, 21 
when the thickness was reduced 0™™-01. 
A plate of mica, 0™™-02 thick, gave for the four transmissions 
80, 76, 39, 26. 
An extremely thin flake was taken from this plate (which was however not 
coloured) : the four transmissions through this flake were, 
86, 85, 61, 46. 
