311 



center of tlie cap. Around H is a three-sided wooden frame I, on whose 

 inner edge is a groove J, dee]) enough to receive one pane of glass. The 



Fit;:. 6. 



pane of glass used is 20 cm. square and must have exactly parallel sur- 

 faces. Light of any desired color may enter the box at H by putting 

 in the groove .1 a glass plate which has been colored in the following 

 way. To a 10 per cent, solution of gelatin add about ."25 of a gram of 

 the ordinary "Diamond Dye" of the color desired, while the gelatin is 

 still hot and in solution. Stir well for a few minutes. Now place one of 

 the glass plates in a perfectly horizontal position in a cool place and pour 

 onto it a thin layer of the colored liquid gelatin. Let stand till the gelatin 

 is solid. In this way colored plates of red, orange, green, blue or any 

 other color or shade of color may be obtained. These colors have the 

 additional advantage of being permanent. The screws K and K^ hold 

 the plates in the correct position in J. By having a number of colored 

 plates any color in the box may be obtained by putting the plate of the 

 desired color in J. 



V. Photometer. 



A simple form of photometer is shown in Fig. 7. It consists of a 

 wooden base A, 110 cm. long and 25 cm. wide, on which stands a rectangu- 

 lar box P>. SI cm. long, IT.-l cm. wide, and 22 cm. high. This box B is 



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Ji 



X 



in rrTTTHTiTiT! 



n 



Fig. 7. 



