Wedge-and- Diaphragm Photometer. 



23 



time different illuminations of the field of comparison for ex- 

 tending the range of the instrument. 



The photometer I have constructed is shown in perspective 

 in the figure. It consists of a brass tube mounted upon a 



stand, and having an eyepiece at one end and a paraffin-lamp 

 at the other. The light of the paraffin-flame, entering at the 

 end of the tube, falls upon a disk of opal glass placed near the 

 middle of the tube, and keeps it constantly illuminated. In 

 the middle, the tube is cut away and a collar fitted over it, 

 which can be turned into any required position. This collar 

 has at one side of it a slit containing a strip of opal glass, 

 opposite which it has fixed to it a frame carrying a wedge of 

 neutral-tinted shade glass, corrected by being combined with 

 a reversed wedge of white glass, with which it is mounted in 

 a slide with rack-and-pinion adjustment. The thicker end of 

 the wedge absorbs about eight times as much light as the 

 thinner end; that is to say, the light from eight candles, after 

 passing through the thicker end, has the same illuminating 

 effect as the light of one candle after passing through the 

 thinner end. 



When the light to be measured, which is placed on the 

 right-hand side of the photometer, is above or below the hori- 

 zontal, the wedge is turned, with the collar which carries it, so 

 that the rays from the light, whatever may be its position, fall 



