850 Mr. H. E. Ives on the 



The apparatus used is that employed in the previous 

 investigations described in this series with certain modifica- 

 tions. An important feature is a special slit, made with both 

 jaws movable. One is moved by the divided head as before 

 (adjustable edge) ; the other is free and may be brought 

 into contact with the first by merely sliding in its ways. 

 The method of use is to leave the prism fixed throughout 

 the experiment ; the adjustable edge of the slit is placed in 

 a position corresponding to the ultra-violet, the free edge 

 brought into contact with it. The adjustable edge is then 

 moved by the divided head until, by the flicker criterion, a 

 piece of the spectrum is transmitted equal in brightness to 

 the comparison standard. The free edge is then brought up 

 into contact with the adjustable edge and another piece of 

 the spectrum of the same brightness exposed by moving the 

 latter. In addition to this slit another means of varying 

 the amount of spectral light is furnished by a variable 

 neutral tint screen * between the light source and the ground 

 glass over the slit. This is used at the end of the spectrum, 

 where the Inst portion of spectral light consists of the whole 

 remainder of the spectrum and cannot be varied in brightness 

 by slit-width variation. (This screen can of course be used 

 at any point if so desired in place of slit-width variation, it 

 being only necessary to take slit-widths of suitable size so 

 as to give approximately the same amount of light.) At the 

 end of the spectral measurements the free jaw is drawn back 

 to its original position, whereupon the eye-slit receives the 

 whole spectrum which it has before received in parts. The 

 prism used in this test was a special constant dispersion, 

 constant deviation type, made by A. Hilger. It gives a 

 considerably shorter spectrum than that furnished by the 

 regular crown glass prism used before. 



A new form of flicker photometer was used in place of the 

 rotating disk of the previous work. This consists of a first 

 surface metal mirror on an oscillating lever. The mirror 

 was made by coating a glass plate with platinum by cathode 

 discharge, then cutting the glass with a diamond. The edge 

 where the glass breaks is almost invisible in the photometer 

 field t- The other end of the lever is driven by a link 

 motion attached to the speed counter axis and having a 

 1-inch stroke. 



This mirror device gives a close approximation to a 

 perfect flicker photometer. "With regard to the question of 



* "A Variable Absorption Screen," Ives, Elec. World, March 16, 

 1912. 

 t Pfimd, Johns Hopkins Univ. Circular, vol. iv. 1906, pp. 20-22. 



