374 C. C. Hutchins — Irregular Reflection. 



The ideal surface imagined above, however, does not exist in 

 fact ; for to be without specular reflection it must possess 

 irregularities large in comparison with the dimensions of the 

 waves of the illuminating ray, and just in proportion as these 

 irregularities are greater does the surface depart from a true 

 plane. 



It will furthermore be seen that a surface, say of very fine 

 grain, might show specular reflection for long waves and irregu- 

 lar reflection for short ones, and consequently, a different dis- 

 tribution of reflected energy according to the length of waves 

 in the illuminating ray. Consequently the results of any 

 investigation of the matter should be given either for definite 

 wave lengths or for the total reflected energy. 



In the following experiments surfaces of various character 

 have been employed, in order to get an idea of the manner of 

 the reflection as determined by the character of the reflecting 

 surface, and at the same time to see how widely the reflection 

 may depart from and how closely it may approximate to 

 theory. 



Apparatus. 



A divided brass circle has at its center a fixed pin, about 

 which revolves an arm 60 cm long, and having an index for 

 reading off the angles. The outer end of the arm has supports 

 for holding the thermograph, which thereby points always to 

 the center of the circle. 



The central pin is drilled for the reception of an upright 

 wire, which latter carries the body under experiment. A num- 

 ber of discs of sheet zinc 4 cm in diameter were provided, and 

 upon these could be spread any body in powder form, by first 

 coating the disc with gum-water. Solid bodies were worked 

 into discs 4 cm in diameter and secured to wires. 



Sunlight is brought into the room by a heliostat having a 

 mirror silvered upon its first surface, and the beam of light is 

 directed upon the body at the center of the circle, and set to 

 receive the beam perpendicularly. An appropriate shutter is 

 introduced into the path of the beam, and can be operated by 

 the observer at the galvanometer by pulling a cord. The 

 thermograph is the same instrument that has been employed 

 in former heat investigations and will be found described in 

 this Journal for May, 1892. 



The controlling magnet of the galvanometer has been in 

 most cases adjusted so as to give the needle a period of about 

 six seconds. 



The method of observation is as follows: The body under 

 investigation and the beam of sunlight being adjusted as above 

 mentioned and the shutter closed, the observer draws the cord 



