1889.] A Compound Wedge Photometer. 



17 



different construction, to which the term Compound Wedge Photo- 

 meter has been applied, and of which the following is a brief 

 description : — 



Two very thin wedges of neutral-tinted glass are made to slide 

 past one another with a uniform rate of motion by the turning of a 

 single milled-headed screw, the idea of the arrangement being dia- 

 grammatically set forth, so far as the wedges themselves are concerned, 

 in fig. 3, where it will be seen that any amount of density, within 



certain limits, can be obtained by equal movement of the two 

 wedges, although a uniformly absorptive area in all parts of the field 

 is rigidly maintained. In the figure, ABC is shown as one wedge, 

 DEF the other, and VIEW the field of view. A cursory inspection 

 of the arrangement at once reveals its most salient advantages, and 

 the fact that any sized disk within the limits of the field of vision 

 will be obscured by the same density of neutral-tinted glass at any 

 and all parts of its image, and hence that the cause of error spoken 

 of as arising from the use of a single wedge is at once removed. 



An instrument so constructed has been subjected to several months' 

 crucial testing, and I have no groundslfor thinking it does not fulfil 

 the requirement for which it was devised. In its final form the 

 arrangement differs from that usually met with as suggested by 

 Professor Prit chard, for it is supplied with a rotating disk . of 

 metal, perforated at intervals to allow the permanent insertion of 

 pieces of neutral- tinted glass of different thickness, each of which 

 can be evaluated for magnitude and used as a constant. Besides, it is 



vol. xlyii. c 



