1886.] Light reflected at nearly Perpendicular Incidence. 285 



error in the comparison of the effects of the reflector and of the disk ; 

 and accordingly (since this may occur in either direction) an 8 per 

 cent, discrepancy in the results is possible. This, however, would be 

 very unlikely, and with a two- or three-fold repetition of the 

 individual settings would be practically out of the question. 



The revolving disks, used to diminish the light on the right hand 

 side of the apparatus in about the same degree as by the mirror 

 under test, were cut from tin plate, about 9 inches in diameter, and 

 carefully centred. The angular apertures were finally calculated 

 from measurements of the chord of the arc, and of the radius. It is 

 important that the disks be thoroughly blackened, in view of the 

 assumption that no light reaches the eye except during the passage of 

 the aperture. Here is one reason why it is desirable to keep the 

 room as dark as possible. The disk should also be properly balanced. 

 On one occasion a curious and at first puzzling effect was observed*. 

 The division line, which should present no visible width, sensibly 

 widened, appearing sometimes darker than the nearly balanced 

 adjoining fields, and sometimes, though more rarely, appearing rela- 

 tively bright. The explanation is to be found in a vibration of the 

 mirror, whose edge forms the division line, in a horizontal direction 

 perpendicular to the line of sight, the vibration being communicated 

 from the revolving wheel through the floor to the table upon which 

 the mirrors stood. It is evident that if the two lights under com- 

 parison were equal, not merely on the average, but at every moment 

 of time, such a movement of the mirror would have no disturbing 

 influence, and could not make the division line visible. But it is 

 otherwise when one of the lights is intermittent, and the vibrations of 

 the mirror are executed (as here they must be) in the same period. 

 For suppose that at the moment when the division line is advanced, 

 so as to invade still further the field from the back mirror, the light 

 is reaching the eye through the aperture in the disk. In this case the 

 parts near the edge of the vibrating mirror will be sending to the eye 

 the full light due to this part of the field. During the remainder of 

 the vibration, no light should reach the eye, but if this mirror 

 retreats, the back mirror sends its continuous light from the same 

 apparent place, so that when the angular opening in the disk is small, 

 it is possible for the part of the field over which the division line 

 vibrates to present an almost doubled brightness, combining in fact 

 the illumination of the two parts of the field. A different phase 

 relation may evidently lead to an abnormal diminution of brightness 

 in the same region. These effects disappeared when the disk was 

 better balanced. 



