564 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



TABLE I. 



Taking the product of the average deflection in centimeters by the con- 

 stant of the torsion balance, the radiation pressure was : — 



2.25 X 4.65 X 10- 6 = 1.05 x 10" 4 dynes. 



The Bolometer. 



To compare the theoretical value of radiation pressure with the above 

 value, it was necessary to measure the energy of the radiation causing the 

 pressure. This was attempted with the aid of a bolometer constructed 

 as follows : — 



On a sheet of platinum 0.001 mm. thick, rolled in silver (by the firm 

 Sy & Wagner, Berlin), a circle P (Fig. ]), 11.25 mm. in diameter, was 

 drawn. The sheet was cut from the edges inward to the circumference 

 of the circle, in such a way as to leave five principal strips A, B, O, D, E, 

 connected to the circle in the manner shown. Other narrower strips, 

 as e, m, n, o, etc., were left to give the disc additional support. The 

 disc, by means of the connecting arms, was mounted with asphalt varnish 

 centrally over a hole, 14 mm. in diameter, bored through a slab S of thin 

 slate. Portions of the silver not to be removed by the acid were care- 

 fully covered by asphalt varnish. Thus on the strips A and B, the silver 

 was protected to the very edge of the circle, while on all the other arms, 

 the silver was left exposed back to the edge of the boring in the slate. 

 The whole system was then plunged into warm nitric acid, and the silver 

 eaten away from all unvarnished surfaces, leaving only the tbin platinum 

 sheet which was blackened by electric deposition of platinum by 

 Kurlbaum's* method. At A, B, G, D, E, holes were bored extending 



* Kurlbaum, Wied. Ann., LXVII. 818 (1899). 



