Dr Searle, Experiments illustrating flare spots, etc. 205 
Experiments illustrating flare spots in photography. By G. F. 
C. Seartez, Sc.D., F.R.S., University Lecturer in Experimental 
Physics, Fellow of Peterhouse. 
| [Read 24 February 1913.] 
— §1. Introduction. When light strikes a refracting surface, it 
‘is partly refracted and partly reflected. For glass of refractive 
index p, the ratio of the intensity of the reflected light to that of 
the incident light is (u — 1)?/(u +1)? for normal incidence and this 
ratio increases as the angle of incidence increases. For glass of 
‘refractive index 1°5 the ratio is 1/25. If the light suffers a second 
‘reflexion, the ratio of the intensity of the twice reflected light to 
‘that of the original light is 1/(25)? or 1/625. For 2n reflexions 
the ratio is 1/(25)”, which diminishes very rapidly as m increases. 
If a luminous point be placed on or near the axis of a lens, an 
‘image will be formed by rays which have passed through the lens 
without suffering reflexion. This is the ordinary image used in 
photography. A second and fainter image will be formed by rays 
which have been twice reflected and there are other images 
formed by rays which have been reflected 4, 6, ... times, but the 
latter images will be very faint unless the source of light is very 
powerful. 
When there are n lenses, the rays which suffer their first 
reflexion at the last air-glass surface can be reflected a second 
time at any one of the 2n—1 air-glass surfaces in front of the 
last surface. The rays which suffer their first reflexion at the 
last surface but one can be reflected a second time at any one of 
the 2n —2 surfaces in front of that surface, and so on. Hence, if 
NV be the total number of images formed by twice reflected rays 
N=(2n—1)+ Qn—2)4+...424+1=n(2n—-1). 
| 
Thus we have the results 
Number of lenses - 1 2 Bo eed ® 
Number of images formed by I & ie Los | 45 
twice reflected rays | cal 
In many photographic lens systems, two or more pieces of 
‘glass are cemented together with Canada balsam. The light 
