Photometry of Lights of Different Colours. 857 



focal length de ; e is a magnesium oxide surface illuminated 

 by the lamp h ; /is a lens of focal length ef. At g is formed 

 an image of the slit c ; the eye placed here sees the image of 

 the diaphragm a in the same plane as the edge of £, and by 

 moving the comparison lamp the field may be made of the 

 brightness of e. The auxiliary lenses are then removed, 

 proper correction made for the loss of light caused by the 

 various lenses, and the slit c used as before. 



A test of the illumination used in the previous investigation 

 of the series — which in turn had been chosen to be in the 

 region where the flicker and equality of brightness method 

 had been found to agree, and where large and small field 

 agree, viz. 300 "illumination units" — showed that lor the 

 writer's eye it was close to 25 metre candles. As this was 

 desirable from various standpoints, it was taken as the 

 constant illumination on e. Each observer was required 

 to find the artificial pupil illumination corresponding to 

 25 metre candles normal pupil, and his luminosity curve 

 was then obtained at that illumination. In this way the 

 results all apply to the standard illumination under ordinary 

 photometric conditions. It was found that the highest 

 illumination used was about twice the lowest. This range 

 would make no noticeable difference in the luminosity curve 

 with the illumination and field size used. 



Details of Procedure. — The measurements of brightness 

 were carried out as in the previous work, by finding the slit 

 openings to match a constant comparison illumination. The 

 zero of the slit was found as follows : — By means of colour 

 screens the light of the comparison lamp was made to match 

 a certain wave-length ("59 /x) ; then a series of three neutral 

 tint screens of known transmission were placed successively 

 over the comparison lamp side and the slit width obtained 

 for photometric match. These values when plotted against 

 the transmissions give a straight line passing through the 

 effective zero. This method proved very satisfactory. 



The calibration for wave-length was provided for by having 

 the eye-slit adjustable sideways by a milled head : a helium 

 tube was placed behind the tungsten lamp under measure- 

 ment so that it could be turned on at intervals during the 

 measurements and the setting of the drum checked. This 

 prevented any instrumental drift. 



A difficulty was met with in carrying the measurements 

 down into the blue of the spectrum, namely, that the 

 necessary slit widths became altogether too great for sensi- 

 bility or accuracy. This difficulty was met by a compromise. 

 For the measurements at '50 /jl and '±8/u, neutral tint screens 



