nutting: brightness of optical images 



133 



The relative brightness of block (without lens) and source /i:7o 

 is the area of the source divided by the square of the distance to 

 the edge (not center) of the source to within 0.1 per cent error. 

 Relative brightness of image and block without lens is the meas- 

 ured ratio Ii.Ii, hence the desired relative illumination of image 

 and object I^'-Iq is obtained by multiplication. Since the com- 

 parison screen of the photometer is illuminated directly by the 

 source, there is no curve due to fluctuations in the source by mat- 

 black walls, and the remainder is measured and corrected for. 



The results obtained on the different lenses are tabulated below. 

 For each test are given the numerical aperture at which the text 

 was made, the relative brightness 7i//o of source and receiving 

 block, relative distance m of image and object, observed relative 

 brightness /2//0, the same quantity computed — neglecting losses 

 b}^ reflection and absorption {B) — and finally the ratio of observed 

 to computed brightness T which is the percentage transmission 

 of the lens. 



If there be no absorption of light within a lens, a transmission 

 of .76 corresponds, for 6 air glass surfaces, to a mean refractive 

 index of about 1.55; hence the higher observed transmissions are 

 quite up to theoretical possibility. 



The first five lenses in the above list were used at various meas- 

 ured apertures. Data are given in the following tables, and 

 in the curves of figure 4. 



