1888 .] Mr G. N. Stewart on Intermittent Light. 
443 
A parallel beam of light was allowed to enter by a small hole in the 
door, and fell upon the rotating mirror, from which it was reflected 
to the fixed one, and thence to the eye of the observer, who stood 
behind the rotating mirror, which in the first experiments he turned 
himself by hand. Afterwards in Owens College, a gas engine was 
used to drive the mirror, on account of the greater steadiness of 
motion. A second feeble beam from a similar source was reflected 
directly to the fixed mirror, without falling on the rotating one. By 
varying the distance of the second source, its reflected image could 
be made of equal brightness to that of the image given by the beam 
falling on the rotating mirror. It served, therefore, as a standard 
of comparison. 
Fig. 1 shows the course of the rays for the position of the 
rotating mirror for which the image falls on the retina. M is the 
rotating, M' the fixed mirror. The interrupted lines show the 
direction of the accessory beam. E is at the position of the eye. 
At E was mounted a blackened tube with a diaphragm, in the 
proper position for receiving the reflected ray. Many details of 
adjustment were necessary, but these it is needless to describe. 
Sometimes a spectroscope wms placed at E with its slit horizontal. 
One half of the slit was illuminated by the interrupted light from 
the rotating mirror, the other half by a fixed light ; and it was 
sought to determine whether any changes in the intensity of 
particular parts of the spectrum took place as the velocity of the 
mirror was increased. For this purpose it was found necessary to 
throw the light directly from the rotating mirror on to the slit of 
