ARTICLES 573 



many purposes accurate determinations of the relative values 

 of wave-lengths are all that is wanted. 



As an example of closeness of repetition and comparison 

 the following may be quoted from the paper by Dershem al- 

 ready referred to. It shows the results of four separate mea- 

 surements of the K series of tungsten (atomic weight 184, 

 atomic number 74). 



TABLE I 



(From Physical Review, p. 473, vol. xi. No. 6) 

 K Series of Tungsten 



These may be compared with the K series shown in fig. 3. 



In this case the width of the slits which limited the inci- 

 dent pencil of X-rays was 0*032 cm., and the thickness of the 

 rock salt crystal, which had been ground to a fine slip, was 

 0*019 cm. The distance apart of the extreme edges of the 

 right and left images on the photographic plate is not actually 

 given in the paper for these particular lines, but would be very 

 nearly 10 cm., and could be measured with great accuracy. 



A method described by Uhler and Cooksey (Physical Review, 

 December 191 7) is novel in certain respects and seems very 

 simple and accurate. 



A photograph is taken first with the plate close up to the 

 crystal as at,P'P' fig. 2, and again when the plate has been 

 moved back to PP, through a distance which can be measured 

 very exactly. Reference points R and R' were obtained by 

 exposing the plates for very short intervals to the direct stream 

 of X-rays, the crystal being removed. 



The order of experiments was — 



(1) Brief exposure at P'P', no crystal, one-half (upper or 

 lower) of the slit being covered. 



(2) Crystal placed in position ; cover removed from slit ; 

 exposure to reflected rays. 



(3) Plate moved back to PP ; further exposure. 



