422 Prof. D. B. Brace on a New Spectrophotometer 



instrument and method of measurement to be described show 

 that quite as great, if not greater, sensibility and greater 

 facility in making comparisons are attainable with a much 

 simpler form. The two chief factors of this system are the 

 peculiar prism used and the method of obtaining the inten- 

 sities free from the errors heretofore involved in the readings 

 of the slit. 



Fig. 1. 



The compound prism (fig. 1) is made up of two equnl rect- 

 angular prisms A D B and ADC polished on their three faces, 

 and so cut that when placed with their longer sides in contact 

 the whole forms an equilateral prism with three polished 

 faces. It is evident that two rays of the same colour, red say, 

 incident at the same angle und in the same plane will pass out 

 and form parallel rays a r if one pass through the prism and 

 the other be totally reflected at d by the face A D. If the 

 incident rays be of a higher frequency, violet say, they will 

 also pass out forming parallel rays a v . Thus two systems of 

 incident parallel rays of white light will form, on emergence, 

 two spectra with corresponding rays exactly parallel, and 

 hence of the same dispersion. If c passes through the face 

 A D and b is reflected, we shall have in a similar way the 

 rays aj and aj forming similar spectra, but in inverse order 

 to the first spectra. This arrangement immediately enables 

 us to realize all the conditions essential for the highest sen- 

 sibility, by obtaining two fields with delimitations which are 

 perfectly sharp, and vanish when set for a match. 



