SCIENCE- GOSSIP. 



67 



now photograph such an object, the different 



opacities of a print, fr our negative, compared 



with the test object will show as very approxi- 

 mately the amount of success obtained. In order 

 to ii'll with any degree of exactness, however, we 

 have (irsfc to measure accurately the luminosity of 

 each of the coloured glasses as compared with the 

 white glass, and then to measure the amount of 

 light passed by each of tin- patches of deposit in 



may be opened or closed whilsl revolving at a high 



rate ul' speed, ami as the sector is divided in 

 degrees, we can measure accurately the difference 

 in the intensity of the two beams. T<< use the 

 instrument we first balance tin- two beams nut il 1 he 

 brightness of the two adjacent patches upon the 

 screen arc equal. Into the path () |' t lm direct beam we 



then place the coloured glass tic- Luminosity of 



which we wish to measure. The iliwrtcil In am 



1 Lumiere 



B 



Lumiere 



A 



Thomas 



° Isochromatic. 



Edwards' 

 ■* Snap-shot 

 Isochromatic. 



Ilford 



5 Chromatic. 



Lumiere 

 ° Panchromatic. 



Cadet t 

 Lightning 

 Spectrum 



Fig. 4. 



cur negative. In order to measure the luminosity 

 of a coloured glass as against white light, we use 

 the whitest light available, that proceeding from 

 the crater of the electric arc. A beam of this 

 white light is allowed to fall as a rectangular 

 patch upon a screen; a portion of this beam 

 is reflected to one side and caused to form another 

 patch close beside the first, a rod being 'placed 

 between the two beams so that each beam shines 

 into the shadow of the rod cast by the other (fig. ">). 

 In this diverted beam is placed a rotating sector ; 

 by a mechanical device the apertures of this sector 



will now appear very much brighter than the 

 coloured beam ; but by reducing the aperture of the 

 sectors, we shall soon reach a point where the 

 reduced light of the diverted beam appears less 

 bright than the coloured patch. By alternately 

 opening and closing the sectors, we shall find a 

 point where the coloured and the white patch 

 appear of the same brightness. We fix the sectors 

 on this point, and, on stopping their revolution, we 

 can read the aperture at which they are set. That 

 such measurements of colour against white light 

 are readable is proved by the consistency between 



1)2 



