740 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
determined of tlie incident light. Glasses can thus be graduated and 
named, e. g. a ground glass of 100 Carcels is one which, brought into a 
field of this intensity, transmits an amount of light exactly equal to one 
Carcel at the unit of distance. 
In making a photometric determination by comparison with a Carcel 
lamp, instead of the distances being changed, the aperture in the screen 
is made variable and its size can be measured by a micrometer screw. 
Colorimeter with Lummer-Brodhun Prism Pair.* — Dr. H. Kriiss 
points out the advantages of the use of four prisms in the colorimeter 
Fig. 88. 
Fig. 89. 
devised by himself over the arrangement of only two prisms which has 
been more recently proposed by Dr. C. Pulfrich. 
* Zeitschr. f. Instrumentenk., xiv. (1894) pp. 283-5. 
