652 On a Form of Cosine Flicker Photometer. 



It must be noted, however, that the principle on which 

 Crova's method depends, only applies rigidly to an incan- 

 descent body yielding a continuous spectrum. 



In the case of a luminescent vapour yielding a spectrum 

 in which the luminosity is concentrated in isolated bright 

 lines, the method is inapplicable. 



Yet, since the portion of the spectrum which Crova's 

 method proposes to utilize is by far the most valuable portion 

 from an illuminating point of view (i. e. the portion which, 

 for a given expenditure of energy, produces the greatest 

 sensation of brightness), the method may prove satisfactory 

 even in many cases of this character. 



The chief difficulty which the author has found in applying 

 this method lies in the fact that, if a sufficient depth of 

 solution is employed to restrict the light passing to a very 

 narrow region of the spectrum, and hence to render the 

 colours of the two illuminated surfaces identical, the lumi- 

 nosity of the surfaces becomes, as a rule, inconveniently low. 

 The only case, in fact, in whieh the author succeeded in 

 obtaining an apparently perfect colour match without undue 

 loss of illumination, was when comparing an arc lamp against 

 an incandescent mantle. 



When a flicker was utilized, however, it was found that a 

 depth of 3 millimetres of solution, which, while most trans- 

 parent to yellow rays, actually allowed rays from X=0'52 to 

 % = 0'6Sfju to pass, rendered the comparison of most sources 

 distinctly easier. 



The following are a few cases in which this method was 

 applied. 



Carbon Glow-lamp compared with Nernst lamp. 



Argand Gas-burner „ „ Incandescent Mantle. 



White Arc-light „ „ Incandescent Mantle. 



White Arc-light „ „ Argand Gas-burner. 



Flame Arc-lamp (Excello carbons) compared with Argand Gas-burner. 



Flame Arc-lamp (Bremer carbons) „ „ Argand Gas-burner. 



In all these cases the means of a set of photometrical 

 results obtained with and without the Crova screen agreed 

 very closely, in the first two cases within 2 per cent. 



The unsteady nature of the arc lights rendered very exact 

 comparisons with these lights impossible, but in this case also 

 the difference seemed to be, at any rate, less than 5 per cent. 



All these sources of light, however, yield a continuous 

 spectrum. It is true that some isolated bright lines, super- 

 imposed over the main spectrum of the arc, serve to increase 

 the efficiency of the flame carbons. Yet the brightest of 

 these lines were congregated in the yellow and orange and 



