RELATIVE VALUES OF LIGHTS. 



101 



intend to study this question fully, for there is no doubt that electricity is 

 becoming more and more adapted to our daily life. 



The measurement of intensity of light is called Photometry, and the 

 instruments used are Pfwtometers. Bunsen's instrument consists of a screen 

 of writing-paper, saturated in places with spermaceti to make it transparent. 

 A sperm candle is placed on one side, and the light to be compared on the 

 other. The lights are provided with graduated bars, and these lights are 

 then removed farther and farther from the screen till the spots of grease are 

 invisible. The relative intensities are as the squares of the distance from 

 the screen. 



Fig. 94. The soap-bubble. 



We append a table showing the comparative cost ol light given by 



Dr. Frankland at the Royal Institution some few years ago. The standard 



of comparison was 20 sperm candles burning for 10 hours at the rate of 

 1 20 grains an hour : 



*. d. s. d. s. d. 



Wax . . 7 2\ Spermaceti . .68 Tallow . .28 



Sperm Oil . i 10 Coal Gas . .04^ Cannel Gas . o 3 



Paraffin . 310 Paraffin Oil . .06 Rock Oil . o 7$ 



There are many other interesting experiments connected with Light, 

 Spectrum Analysis, etc., etc., all of which we will defer for a time until we 

 have examined the Eye and some effects produced upon it by Light, illus- 

 trated by numerous diagrams in the pages next following. 



