VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 753 



in 2 prisms of different glass, when the angles are such as to give the same mean 

 refraction, the dispersive power is known to differ. Their transmissions have been 

 still less ascertained ; and I need not add, that the internal and external reflec- 

 tions, and the scattering of rays on every one of the surfaces, are all of such a 

 nature as must throw some obscurity on every result of experiments made with 

 prisms. A lens partakes of all the inconveniencies of the prism ; to which its 

 own defects of spherical aberrations must be added. And a mirror, besides its 

 natural incapacity of separating the rays of light from the different sorts of heat, 

 scatters them very profusely. But if we have been scantily provided with mate- 

 rials to act on rays, it has partly been our own fault : every diaphanous body may 

 become a new tool, in the hands of a diligent inquirer. 



My apparatus for transmitting the rays of the sun is of the following construc- 

 tion: see fig. 13, pi. 13. In a box, 12 inches long, and 8 inches broad, are 

 fixed 2 thermometers. The sides of the box are 2^- inches deep. That part of 

 the box where the balls of the thermometers are, is covered by a board, in which 

 are 2 holes of * inch diameter, one over each of the balls of the thermometers ; 

 and the bottom of the box, under the cover, is cut away, so as to leave these balls 

 freely exposed. There is a partition between the 2 thermometers, in that part 

 of the box which is covered, to prevent the communication of secondary scat- 

 terings of heat. Just under the opening of the transmitting holes, on the outside 

 of the cover, is fixed a slip of wood, on which may rest any glass or other object, 

 of which the transmitting capacity is to be ascertained. A thin wooden cover is 

 provided, fig. 14, that it may be laid over the transmitting holes, occasionally, to 

 exclude the rays of the sun ; and on the middle of the slip of wood, under the 

 holes, a pin is to be stuck perpendicularly, that its shadow may point out the 

 situation of the box with respect to the sun. The box, thus prepared, is to be 

 fastened on 2 short boards, joined together by a pair of hinges. A long slip of 

 mahogany is screwed to the lowest of these boards, and lies in the hollow part of 

 a long spring, fastened against the side of the upper one. The pressure of the 

 spring must be sufficiently strong to keep the boards at any angle ; and the slip 

 of mahogany long enough to permit an elevation of about 85°. 



In order to see whether all be properly adjusted, expose the apparatus to the sun, 

 and lift up the board which carries the box, till the directing pin throws the shadow 

 of its head on the place where the point is fastened. Then hold a sheet of paper 

 under the box, and, if the thermometers have been properly placed, the shadow 

 of their balls will be in the centre of the rays passing through the transmitting 

 holes to the paper. A screen of a considerable size, fig. 15, with a parallelogram- 

 mic opening, should be placed at a good distance, to keep the sun's rays from 

 every part of the apparatus, except that which is under the cover; and no more 

 sun should be admitted into the room, than what will be completely received on 

 the screen, interposed between the window and the apparatus. 



vol. xviii 5 D 



