160 LIGHT AND PROTOPLASM [Ch.VII 



easy to determine the intensity of white light in terms of a 

 recognized unit ; namely, a paraffine candle burning at the rate 

 of 7.78 grammes per hour. A paraffine candle burning at this 

 rate has one candle power (C. P.) ; burning at twice this rate, 

 2 candle power, and so on. A comparison of any other light 

 with this standard may be made by means of any of the well- 

 known photometers, of which text-books of physics give a 

 description. 



The determination of the intensity of a colored light requires 

 an additional piece of apparatus ; namely, a spectrophotometer. 

 The two principal types of spectrophotometer are that of 

 ViEROEDT ('73) and that of Glan ('77), both of which 

 have undergone important improvements. The principle in 

 both types is the same. A spectrum of both the unmodified 

 (standard) light and that which has passed through the colored 

 screen are made side by side, so that their corresponding colors 

 can be compared. Since the source of light is the same, every 

 part of the spectrum of the unmodified light will be brighter than 

 the corresponding part of the spectrum of the colored light. 

 To bring the corresponding colors in the two spectra to the 

 same intensity, the unmodified light must be made less intense 

 to a measurable extent. In Vibeoedt's spectrophotometer 

 this result is brought about by narrowing that half of the slit 

 through which the unmodified light passes to get to the prism. 

 In Glan's apparatus the diminution in intensity is gained by 

 the polarization of both lights and the obscuring of the brighter 

 by the rotation of its analyzing NiCHOL prism, until equality 

 of brightness is obtained. A modified form of Vieeoedt's 

 convenient instrument is made by H. KEtJSS of Hamburg, 

 Germany. A modified form of Glan's photometer is de- 

 scribed by VoGEL ('77). 



Vogel's apparatus (Pig. 43) consists essentially of a collimator contain- 

 ing (1) a slit of changeable width, separated by a band q into an upper and 

 a lower part to receive respectively the modified and the normal light ; (2) a 

 lens to render the rays parallel before they impinge upon (3) a doubly refract- 

 ing quartz prism, by which both upper and lower rays are broken into two 

 polarized rays. Of these four rays the uppermost and the lowest are cut 

 off by a diaphragm near F, so that only the middle two, which lie near 

 together, pass eventually to the eye. These two rays are oppositely polar- 

 ized and come, one from the upper, the other from the lower slit. The two 



