LETTER OF J. HENRY TO REV. S. B. DOD. 155 



that, among a large number of transparent solids, some were 

 permeable to the phosphorescing agent, and others impermeable 

 or imperfectly permeable. Among the former were ice, quartz, 

 common salt, alum. Among the latter glass, mica, tourmaline, 

 camphor, etc. Among liquid permeable substances were water, 

 solutions of alum, ammonia; while among the impermeable liquids 

 were most of the acids, sulphate of zinc, sulphate of lead, alcohol, 

 etc. 



It was found that the emanation took place from every point of 

 the line of the electric discharge, but with more intensity from the 

 two extremities; and also that the emanation producing phosphor- 

 escence, whatever be its nature, when reflected from a mirror obeys 

 the laws of the reflection of light, but no reflection was obtained 

 from a surface of polished glass. It is likewise refracted by a prism 

 of rock salt, in accordance with the laws of the refraction of light. 

 By transmitting the rays from an electrical spark through a series 

 of very thin plates of mica, it was shown that the emanation was 

 capable of polarization, and, consequently, of double refraction. 



IX. The next series of investigations was on a method of deter- 

 mining the velocity of projectiles. The plan proposed for this 

 purpose consisted in the application of the instantaneous transmission 

 of the electrical actjon to determine the time of the passage of the 

 ball between two screens, placed at a short distance from each other 

 in the path of the projectile. For this purpose the observer is pro- 

 vided with a revolving cylinder moving by clock-work at a uniform 

 rate, and of which the convex surface is divided into equal parts 

 indicating a fractional part of a second. The passage of the ball 

 through the screen breaks a galvanic circuit, the time of which is 

 indicated on the revolving cylinder by the terminal spark produced 

 in a wire surrounding a bundle of iron wires. Since the publica- 

 tion of this invention various other plans founded on the same 

 principle have been introduced into practice. 



X. Another series of experiments was in regard to the relative 

 heat of different parts of the sun's disk, and especially to that of the 

 spots on the surface. These were made in connection with Professor 

 S. ALEXANDER, and consisted in throwing an image of the sun on 

 a screen in a dark room by drawing out the eye-piece of a telescope. 



