OF ARTS AND SCIENCES. 389 



XIII. 



COXTRIBUTIOXS FROM THE PHYSICAL LABORATORY OF 

 THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



E. C. PICKERING, Peofessok of Physics. 



No. II. — LIGHT TRANSMITTED BY ONE OR MORE PLATES OF 



GLASS. 



By W. W. Jacques. 



Kead, April 13, 1875. 



The following experiments were made for the purpose of determining 

 the jDercentage of light transmitted through I, 2, ..., 10 plates of glass, 

 normal to the direction of the light, and of one, four, and ten plates 

 when i was 0°, 5°, ..., 05°. 



The apparatus used consisted of a triangular frame, isosceles and 

 right angled, having a periphery of 100 inches. A gas jet was placed 

 at the right angle, and two mirrors were so placed at the other angles 

 as to reflect the light from the jet along the hypothenuse, thus giving 

 the effect of two equal sources of light 100 inches apart. The plates 

 of glass were mounted on a graduated circle placed between the jet 

 and one of the mirrors, and the light cut off was measured by a Bunsen 

 disc, movable along the hypothenuse of the triangle. (See " Physical 

 Manipulation," Expt. G7. Pickering.) 



The plates used were of common 12 X 18 window glass, and were 

 carefully cleaned with rotten-stone, and then dried by rubbing with 

 chamois skin immediately before each experiment. 



The experiments were made in a dark room, whose walls were 

 painted black, and it was found that the reflection from a sheet of 

 paper, or even from the clothes of the observer, was sufficient to pre- 

 vent the accurate setting of the disc. The following tables give the 

 results of the experiments ; each number being the mean of four obser- 

 vations, and the probable error of a single observation being 0.42 of 

 one per cent. 



Table I. gives the percentage of light transmitted by I, 2, ..., 10 

 plates when i= 90°. The first column gives the number of plates, the 



