214 Dr. Fleming : Ratio between Mean Spherical and 



space between the photometer and the lamp into three nearly 

 equal parts. 



The tubular lamp had a protractor attached to it, and could 

 be tilted over at various angles to the vertical line, keeping it 

 in one vertical plane normal to the photometer-disk. It is 

 obvious that i£ the illumination sent out from unit of length 

 of the straight filament obeys the cosine law above mentioned, 

 then tilting the lamp as described to various angles should 

 make no difference in the observed illumination on the 

 photometer. As a matter of fact, the observed illumination 

 decreased rapidly as the lamp was tilted over, in accordance 

 with the figures in the following table. 



Angle between the straight Observed candle-power or 



filament and the plumb-line illumination through the 



through its centre. two baffle-screens. 



2-33 



15 2-27 



30 223 



15 2*15 



60 1-96 



75 1-06 



It is clear, however, that some part of the decrease of light 

 at high inclinations is due to the loss of light by reflexion at 

 the inner surface of the tubular glass bulb. To obtain the 

 correction for this reflexion a sheet of glass was interposed 

 between an ordinary lamp and the photometer, the plane of 

 the glass being tilted at various angles to the plane normal to 

 the ray. The photometric readings were as follows : — 



Angle at which sheet of 



glass was placed to the Observed intensity 



plane normal to the ray. of the light. 



26-1 c.p. 



15 25-9 „ 



30 25-8 „ 



45 25-2 „ 



60 25-4 „ 



75 16-7 „ 



It follows therefore, that if the carbon filament in the 

 tubular lamp could have been used without a glass envelope, 

 the light sent by it through the baffle- screens would have 

 been greater in the ratio of the first term in the right-haud 

 column of the previous table to any succeeding term in the 

 same column corresponding to the same angle of inclination 

 of the filament. 



If therefore we call the intensity of the light sent through 

 the screens unity when the filament is vertical, then its true 



