S. JR. Williams — An Achromato scope. 109 



(c) Tests for substances to be used in Achromatic lenses. 



This paper wishes to point out particularly the effectiveness 

 of spectral bands in studying transparent substances in order 

 to find achromatic combinations. The method employed here 

 has especial significance when applied to thin films of various 

 kinds of glass in order to select combinations which will pro- 

 duce the best achromatic lenses. Preliminary work in testing 

 the achromacy of various liquids would indicate that for some 

 purposes liquid lenses might be used to a great advantage. The 

 liquids would have to be held in glass containers which were 

 ground to the proper curvature, but whose walls were parallel 

 surfaces. 



The arrangements adapted to testing substances for achromatic 

 combinations have been called achromatoscopes because with- 

 out the necessity of measuring the refractive indices we can 

 tell by inspection whether there is achromacy or not. The 

 various systems consist essentially of devices for viewing two 

 spectra in juxtapo ition in which one is a channeled spectrum 

 from one film and the other is from another thin film. Any 

 comparison spectrometer may be used. 



If the interference bands in both spectra are in coincidence 

 throughout the spectra then perfect achromatization exists, for 

 the condition of achromacy is fulfilled, viz., that 



This condition is shown in fig. 1 for that section of the spec- 

 trum between the C and .D lines where the channeled spectrum 

 from a thin film of absolute alcohol is compared with a solu- 

 tion of fuchsine and absolute alcohol (concentration l:0 - 000072 

 by weight). Perfect achromatization may be present even 

 though the interference bands are not in coincidence. The 

 necessary and sufficient condition is expressed in (13). When 

 coincidence or approximate coincidence occurs it will be found 

 much easier to determine whether achromatization is present 

 or not. In fig. 2 is shown a comparison of channeled spectra 

 from two mica films. The bands are not in coincidence but 

 (13) does hold. It will be evident from a comparison of figs. 

 1 and 2 that any departure from achromatism can be detected 

 most readily when the nearest approach to coincidence of bands 

 is sought for. 



The best mechanical arrangement for comparing channeled 

 spectra is that shown in fig. 7. A slit, S 15 is illuminated with 

 sunlight so that in the resulting spectra, wave lengths may be 

 determined from the Fraunhofer lines. By means of a lens, 

 L 1? and two right angled prisms, p i and p 2 , placed on top of 



