8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 93 



direct sunlight. The observations were made 66 feet from the filter 

 with the filter temperature held at 20° C. This purity of color is ob- 

 tained over a 6-inch circular area at that distance. 



AN IMPROVEMENT THAT PERMITS THE USE OF AN INTENSE 



BEAM OF LIGHT 



The necessity of an optical system and the disadvantage of the 

 temperature coefficient have been pointed out in detail in previous 

 publications. However, for any exacting use of the filter where much 

 light energy is used there is still another trouble which proves to be 

 very serious : when a strong beam of light is forced through the filter, 

 some energy is absorbed, and the center reaches a higher temperature 

 than the edges, owing to poor heat conduction. The color transmitted 

 is no longer pure, even when the filter is in a water bath. The writer 

 has finally overcome this difficulty by inserting aluminum vanes 

 through the body of the filter so as to cut ofif the least amount of 

 light and carry off as much heat as possible. Details of a satisfactory 

 filter equipped with these vanes are shown in figure 4. The body of 

 the filter is cast aluminum, machined as shown, to take the glass 

 windows and the necessary gaskets. No cement is known to the writer 

 that will satisfactorily withstand the benzene and carbon disulphide 

 mixture on the inside and the water on the outside. For this reason 

 the windows were clamped on, as shown, with a soft lead gasket (-^-^ 

 inch thick) underneath the glass. Ridges were machined on the 

 aluminum face to press into the lead gasket and improve the seal. A 

 iV-iiich rubber gasket is placed between the glass window and the 

 brass clamping ring. The vanes are of g^-inch aluminum assembled 

 so that their extremities press firmly against the inner wall of the 

 aluminum case, thus providing a path of good heat conduction from 

 the inside of the filter to the surrounding water bath. The holes 

 shown in the vanes allow the cell to be filled with the glass particles 

 after it is assembled. Without these vanes, the center of this filter 

 rose 9° C. above the temperature of the water bath when the rays from 

 a i.ooo-watt lamp were concentrated on the filter. With the vanes 

 installed, the temperature at the center of the filter rose only 0.25° C. 

 above that of the water bath under the same conditions. 



Plate 2 figure i, is a photograph of this filter in its water bath. 

 The filter is filled with glass particles and a liquid (about 9 percent 

 carbon disulphide in benzene), and gave the transmission curve shown 

 in figure 3 under the conditions previously mentioned. Plate 2, fig- 

 ure 2, is a photograph of a 12 x 14-inch filter (not filled) with its water 

 bath. When in use the water of the bath is thermostated and stirred. 



