lxxv 

 February 5, 1872. 



The President in the chair. 

 Twelve members present. 



Dr. Charles O. Curtman made a communication on " Light 

 in vacuo." 



After a few introductory remarks on the close relation of the different 

 dynamical agents to each other, and the production at will of any mode 

 of motion from the others, the subject of light was briefly discussed. Its 

 compound nature, marvelous velocity in traversing space, and still more 

 wonderful quickness and minuteness of oscillation. Then the solar spec- 

 trum was explained and compared with that derived from various terres- 

 trial sources, especially that of electric light. The visible part of the 

 spectrum, consisting of various degrees of refrangibility, color, wave, 

 length, and velocity, was shown to be accompanied by invisible spectra of 

 rays called respectively calorescent, which exist beyond the least refran- 

 gible red, and fluorescent, beyond the most refrangible violet. Modes 

 were pointed out for making visible those rays which, under ordinary 

 circumstances, do not affect our eye, and which must be brought to oscil- 

 late synchronously with the visible rays before being seen as variously 

 colored light. The electric light being very rich in fluorescent rays is well 

 adapted for experimenting with them, being so easily controlled by the 

 operator. 



A number of experiments were then made, illustrating the foregoing 

 remarks. The electro-magnetic force was applied to the production of 

 mechanical motion in various ways. By means of the Ruhmkorfs In- 

 duction apparatus, long and powerful sparks were at first transmitted in 

 open air. Afterwards, through tubes of several feet in length, exhausted 

 of air by means of a large air-pump, they were shown to increase amaz- 

 ingly in length and in brilliancy of color. The tube was filled with tur- 

 pentine vapor instead of air and again exhausted, when the light lost 

 most of its red rays and became almost white, and stratification of alter- 

 nate luminous and dark portions was plainly visible. Afterwards, a 

 number of tubes were exhibited, which had been previously prepared by 

 filling them with various gases and vapors, and after almost complete ex- 

 haustion, had been permanently sealed, the communication with the 

 interior space being effected by platinum and aluminum wires — Gessler 

 tubes. Into these tubes were also introduced pieces of Uranium glass, 

 or they were surrounded by glass capsules filled with various fluorescent 

 solutions, so as to exhibit the different phenomena of fluorescence. 

 Others contained some of Becquerel's phosphorescent sulphides, which 

 continued to emit colored light after cessation of the primary illumina- 

 tion. The remarkable difference between the luminous appearances at 

 the positive and negative poles was rendered strikingly distinct by vari- 

 ous tubes and by changing the direction of the electric current. Tubes 

 with diaphragm in centre, having both ends filled with different gases. 



