proceedings: philosophical society 181 



connecting refractive index and density'. The "refraction ratio," or 

 ratio of the several vahies obtained for each substance, has been com- 

 pared with the cr3rstallographic axial I'atio in each case. In some 

 substances, as urea and iodoform, the two sets of values show exact 

 inverse proportionality, and it is concluded that this indicates that the 

 luunljer of pinacoidal planes in the two cr^'stallographic directions in 

 the crystal molecule, or unit cell of the space lattice, is the same. 

 Others, like oxahc acid, show inverse proportionalitAMU two directions 

 but not in the third, although in the latter a simple integral relation 

 exists. It is concluded that in these the crystal molecule contains dif- 

 ferent numbers of planes in some diiections than in others. In acet- 

 amide, which is dimorphous, the planes show somewhat greater diver- 

 gence in the unstable than in the stable form, suggesting that there is a 

 tcndenc}^ toward equalization of the number of planes. From these 

 data it is possible to draw conclusions as to the types of space-lattices 

 represented in these substances, without the necessity of submitting 

 theni to examination by X-rays. 



Discussion: The paper was discussed by Messrs. Merwin, White, 

 SosMAX, and Bichowski. 



A paper by W. F. Meggers and C. G. Peters on The refractive index 

 and optical dispersion of air, was presented by Air. Peters. This 

 paper was illustrated by lantern slides. A survey of previous re- 

 searches on refraction of air shows that most investigators have worked 

 either with white light or with one monochromatic radiation, and dis- 

 persion measurements have been limited to a small interval of the 

 spectrum. No index measurements exist for waves longer than those 

 corresponding to orange light, and in the ultra-violet the dispersion 

 formulas disagree by more than 10 per cent of the index of refraction. 



Recent work in spectroscopy makes it very desirable to have more 

 accurate and extensive data on the index of refraction and dispersion of 

 ail'. The international system of standard wave length measurements 

 made under other conditions require small corrections because of the 

 effect of temperature and pressure of the air upon its optical dispersion. 

 Furthermore, it is often desirable to multiply wave lengths measuied in 

 air b}' the indices of refraction of air for these wave lengths and thus 

 convert them to their value in vacuum. An accuracy of one part in 

 several millions is now striven for in the measurement of wave lengths, 

 and to maintain their relative accuracy in the reduction to vacuum 

 values it is necessary to know the indices of refraction to about one unit 

 in the seventh decimal place. 



For several years the Bureau of Standards has been engaged in the 

 accurate measurement of wave lengths. Interferometer comparison of 

 wave lengths have been made throughout a large range of spectra antl 

 the grating spectra of more than fifty of the chemical elements have been 

 photographed and measured in the red and infra-red spectral regions. 

 In connection with these accurate measurements of wave lengths, it 

 was thought advisable to measure the absolute indices of i-efraction of 

 air for the entire spectra region that is accessible to photography. 



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