2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 93 



and blue light when at 50° C. Konrad von Fragstein, in 1932 and 

 1933, describes a filter for the near ultraviolet. One filter covers the 

 range from 3000 A to 3700 A by temperature variation. E. Knudsen. 

 in 1934, discusses all the various ways of making these filters and 

 points out the possibility of making a filter of particles of low-dis- 

 persion glass in combination with particles of high-dispersion glass 

 fused together, both having the same index of refraction for the 

 desired wave length. He has made such a filter but gives no details 

 of its performance. 



DESCRIPTION OF THE FILTER AND DISCUSSION OF 

 ITS ACTION 



In their commonest form these filters are made up of a solid pack 

 of optical glass particles (0.5 to 2 mm in size) in a glass cell, with 

 the spaces between filled with a liquid having the same index of refrac- 

 tion as the glass for the wave length desired. (The present paper is 

 not concerned with the various emulsions and colloidal preparations 

 exhibiting " Christiansen colors." Readers interested in these are re- 

 ferred to Knudsen, 1934.) Figure i gives the curves — index of re- 

 fraction plotted against wave length — for a low-dispersion (borosili- 

 cate) crown glass and a suitable liquid — 10 percent (by volume) car- 

 bon disulphide in benzene at 20° C. (both anhydrous). Remembering 

 the laws of refraction and reflection at an interface, we see that for 

 the wave length where both liquid and glass have the same index of 

 refraction, the filter acts as a solid plate, and the rays of this wave 

 length are transmitted without deviation or reflection loss within the 

 filter. All other rays of shorter and longer wave lengths are deviated 

 and reflected in an amount dei)endent upon the difiference in the in- 

 dices at the interfaces — glass to liquid and liquid to glass. Examining 

 these curves in figure i more closely, we see that they depart from each 

 other more rapidly on the blue side of the crossing than they do on 

 the red side. This is typical of most suitable glasses and liquids. This 

 shows that the filters will have a sharper blue " cut-off " than the red. 

 Also a filter made for blue light will transmit purer colors than one 

 made for longer wave lengths. These two characteristics are evident 

 in the curves shown in figures 2 and 3. Obviously, it is desirable to use 

 a glass of the lowest possible dispersion in combination with a liquid 

 having the highest possible dispersion. 



The refractive index of a liquid changes rapidly with its tempera- 

 ture in comparison with that of the glass. Hence the color transmitted 

 by the filter will vary with its temperature. Thus to maintain a given 

 color, the temperature of the filter must be held constant. For use 



