528 Dr. Herschel's Experiments on the solar, and 



Table of the transmission of terrestrial scattered Light through 

 various Substances ; 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 transmitted 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 followed the example of an eminent optical 

 writer, who does not so much as hint at a possibility that there 

 may be a difference. Before I describe my apparatus, I ought 

 to mention that it is intirely 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 moveable. In the following construction, the 

 greatest difficulties have been removed ; and a desirable consist- 

 ency in the results of the experiments, when often repeated, 

 has now been obtained. 



A board about fourteen feet long, and six inches broad,*f has 

 two slips of deal, an inch square, fastened upon the two sides : 

 these make a groove, for two short pieces to slide in, backwards 

 and forwards. The two sliding pieces J carry each a small 



* See Traite d'Optique, page 16, Fig. 5 ; Ouvrage postbume de M. Bougubr. 

 f See Plate XXVI. Fig. 1. J See Fig. 2 and 3. 



