lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 93 



to 2 mm (usually graded closer — i. e., 0.5 to i mm, or i to 2 mm). 

 A cell 15-mm thick usiug the 0.5 to i mm particles gives approximately 

 the same results as a 30-mm cell using the i to 2 mm particles. With 

 a given optical system, reducing the particle size or increasing the cell 

 thickness gives a narrower transmission curve with lower percentage 

 transmission at the " pealc." In an " ideal " filter the glass particles 

 would be perfectly homogeneous as to refractive index, and these par- 

 ticles and the liquid surrounding them would be all at exactly the same 

 temperature. It is because these two conditions can never be realized 

 that the percentage transmission at the " peak " decreases as the num- 

 ber of interfaces in the filter is increased. 



In filling the cell it is best to put the liquid in first — enough to fill the 

 cell about half full. The glass particles are then poured in slowly so that 

 air bubbles are not carried down. It is difficult to free the cell of air 

 bubbles after it is packed solid with the glass particles. The liquid or 

 liquids used must be anhydrous and of the highest purity. The mixing 

 of carbon disulphide and benzene — originally suggested by Chris- 

 tiansen in 1884 — to obtain a liquid of any desired index of refraction 

 (between that of pure benzene and pure carbon disulphide, of course) 

 has been found very satisfactory by the writer in spite of its relatively 

 high temperature coefficient. Methyl benzoate, used by Weigert (1929, 

 1930), in combination with crown glass particles makes a remarkably 

 variable filter. Von Fragstein (1932, 1933) uses a mixture of 44 per- 

 cent alcohol (ethyl) and benzene with fused quartz particles for an 

 ultraviolet filter. This filter, with suitable optics, transmits a narrow- 

 band of wave lengths in any desired part of the region 3000 A to 

 3700 A. The wave length of maximum transmission is shifted as 

 desired in this region by temperature variation, just as in Weigert's 

 methyl benzoate cell. 



Various optical systems have been described in the literature. Wei- 

 gert's (1929) autocollimator is of considerable interest as it passes 

 the rays twice through the filter. The writer has shown that the filter 

 can be used successfully without an optical system (other than plane 

 mirrors and diaphragms) in direct sunlight, or, of course, in any beam 

 of similar parallelism of rays. In using an optical system it is again 

 emphasized that the diaphragm at the image of the source of light must 

 conform in size and shape to this image. Any change in this diaphragm 

 will change the transmission characteristics of the set-up. This is 

 shown clearly by Weigert (1929, fig. 13, p. 159). 



In the use of the filter for studying the wave-length effect of some 

 photochemical phenomena it is necessary to allow for or take into 

 account the eftect of the " undesired " colors — i. e., those wave lengths 



