Simplified Dispersion-Photometer. 4? 



light ; that is, the reading of the pointer was not merely a 

 reading of its distance from the screen, but it was a reading in 

 standard candles of the power of the light. Three such scales 

 were placed on the instrument; and there were three certain 

 distances at which the lamp had to be placed for examination. 

 The tilting-arrangement was of course different from that of 

 the earlier forms. 



As the instrument had by this time (the end of last year) 

 come into a rather extensive practical use, we had opportu- 

 nities of seeing that, as an instrument to be used by unscien- 

 tific persons, it was not yet in a perfect condition, in spite of 

 the many changes that had been made in its construction. 

 The most important difficulty was due to the fact that a slight 

 lateral change in the position of the observer's eye caused the 

 apparent illumination of the screens to vary. Being aware of 

 this fact ourselves, we maintained a certain fixed position of 

 the eye when making observations ; but the instrument could 

 not at once be used by persons not accustomed to make deli- 

 cate experiments. 



5. The fifth form, which we now present to the Society, 

 is the outcome of our labours on this subject. We have all 

 along seen the disadvantage of using the Bouguer's two- 

 screen method, since, when lights are examined that have 

 passed through tissue or tracing-paper, a very slight change 

 in the position of the observer's eye makes a very great differ- 

 ence in the apparent illumination, whereas, using Bumford's 

 method, when a sheet of white blotting-paper is employed as 

 a screen very considerable changes in the position of the eye 

 produce no change in the apparent illumination — a result, 

 however, which is not attainable when ordinary drawing-paper 

 is used as the screen. If, however, Bumford's method is to 

 be used to measure the rays coming at different angles from 

 an electric light, a mirror must be employed to reflect them 

 successively onto the same screen; and if used in the ordinary 

 way, the angle of incidence of the rays on the mirror will be 

 different in different cases. Now the difficulty that always 

 met us arose from the inequality of the reflecting-power of an 

 ordinary mirror for rays falling on it at different angles of 

 incidence. We have, however, completely overcome this dif- 

 ficulty in an extremely simple way, by causing the mirror to 

 turn about a horizontal axis inclined at 45° to its plane, and 

 the whole photometer to turn about a vertical axis. With this 

 arrangement the angle of incidence, and consequently the 

 proportional absorption, is the same whatever be the inclina- 

 tion of the rays coming from the lamp to the mirror ; and, 

 further, the angle being 45°, the amount of rotation of the 



