648 Mr. J. S. Dow on a Form of 



Beyond these limits, however, the agreement was naturally- 

 less satisfactory. Hence it seems desirable, when using an 

 instrument of this type to assign convenient distances to the 

 sources of light and to utilize the most open, central, portion 

 of the scale. 



When using a photometer with inclined photometric sur- 

 faces such as occur in the Ritchie wedge, attention must be 

 paid to the possibility of "angle-errors," i.e. errors introduced 

 by uncertainty as to the exact angle at which the rays of 

 light strike the illuminated surfaces. In this case a consider- 

 able rotation of the photometer, as a whole, about a vertical 

 axis will alter the illumination of both surfaces equally and 

 will, therefore, not affect the photometric result. But if the 

 two sources are not in true alignment with the photometer, 

 or if the photometer is tilted slightly about a horizontal axis, 

 the illumination of the photometric surfaces may be differently 

 influenced thereby, and errors may result. 



Suppose, for instance, that we are comparing the intensities 

 of two glow-lamps each half a metre distant from the photo- 

 meter. Then, if the centre of illumination of one of these 

 sources is raised say, one centimetre, the angle at which the 

 rays from this source strike the surface presented to it is 

 altered by about 0*6 degrees. This corresponds to an 

 alteration of illumination of about one per cent., when the most 

 open, central portion of the scale is used. 



It is desirable, therefore, that the sources of light should 

 be distant not less than, say, 1 metre from the photometer. 

 It should then be possible to adjust the alignment of the 

 lamps with sufficient exactitude to avoid appreciable error 

 from this source. Indeed, in making accurate experiments 

 with any photometer, it is desirable that the distance of the 

 sources from the photometer should not be less than this value. 

 Otherwise the inverse square law cannot be rigidly applied 

 owing to the fact that the centres of illumination are rarely 

 correctly located*. 



A tilt of only one degree of the photometer, as a whole, 

 about a horizontal axis would cause an error of about 4 per 

 cent., even when the most favourable portion of the scale is 

 used. It is, therefore, desirable that no appreciable tilt of 

 this character should be introduced either by inequalities in 

 the level of the bench or undue play in the carriage of the 

 photometer. 



Yet, it may be pointed out that the " double comparison 



* ' Electrician/ Sept. 14th, 1906, for some notes by the author on 

 this point. • 



