VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 783 



with an olive colour burnt into it, stops 984 rays of light, or 120 of heat, and 637 

 invisible rays, or 559 °f heat. The sum is 679 which that glass should stop, 

 but it stops actually 79*2 ; so that, as in the foregoing instance we had a deficiency 

 of 242 rays, we now have an excess of 113; which plainly shows, that no hypo- 

 thesis of any other proportion between the visible and invisible rays of heat can 

 answer to both cases; and that consequently, not only the present, but every other 

 assumption of this kind, must be given up as erroneous. 



I shall not enlarge on these arguments, as I take them to be sufficiently clear to 

 decide the question we have had under consideration. I also forbear going into an 

 examination of what our 6th article, which treats of scattered heat, might afford, 

 in addition to the former arguments. It may just be remarked, that the 211th 

 experiment points out a black object, which scatters more heat than a white one ; 

 while the case, as to light, is well known to be the reverse. The 21Qth experi- 

 ment also shows, that the scattering of heat of gold paper is considerably inferior 

 to that of black velvet ; whereas a contrary difference, of a very great extent, is 

 pointed out between these two substances ; for black velvet scatters only 7 rays of 

 light, while the scattering of gold paper amounts to more than 124000. I am 

 well aware that this difference will perhaps admit of a solution on other principles 

 than those which relate merely to the laws of scattering, and confess that many 

 experiments are still wanting to complete this article, which cannot now be given ; 

 but as this paper is already of an unusual length, I ought rather to apologize for 

 having given so much, than for not giving more. 



Table of the Transmission of Terrestrial Scattered Light through various Sub- 

 stances ; with a short Account of the Method by which the Results contained in this 

 Table have been obtained. — The transmissions here delivered are called terrestrial 

 and scattered, to distinguish them from others, which are direct and solar; and, 

 in the use I have made of them in the foregoing paper, it has been supposed that 

 light-making rays, whether direct and solar, or scattered and terrestrial, are trans- 

 mitted in the same manner ; or that the difference, if there be any, may not be 

 considerable enough to affect my arguments materially. In this I have only fol- 

 lowed the example of an eminent optical writer, who does not so much as hint 

 at a possibility that there may be a difference. Before describing my apparatus, I 

 ought to mention that it is entirely founded on the principles of the author now 

 alluded to,* and that no other difficulty occurs in the execution of his plan, than 

 how to guard properly against the scatterings of the lamp : for the light which 

 this will throw on every object, must not be permitted to come to the vanes; since 

 these scatterings cannot remain equal on both vanes, when one of them is move- 

 able. In the following construction, the greatest difficulties have been removed ; 

 and a desirable consistency in the results of the experiments, when often repeated, 

 has now been obtained. 



* See Traite d'Optique, page 16, fig. 5 j Ouvrage posthume de M. Bouguer.— Orig. 



