270 Mr Newall, On the marks made by stars etc. [Feb. 22, 



Of these photographs I propose only to speak of two. In the 

 taking of one of them, the aperture of the object-glass was re- 

 duced to 24 inches, and an opaque circular disc, 12 inches in 

 diameter, was suspended in front of the middle of the object- 

 glass, so that light was prevented from falling on the central 

 part. The telescope was then pointed to a bright star and a 

 photographic plate was exposed near the focus, at a distance 

 1J inches within the visual focus (i.e. nearer to the object-glass). 

 The appearance on the plate, which was sensitive to the yellow 

 and green especially between \ 5900 and A, 5200, is mainly due 

 to the light focussed at the visual focus ; and the mark produced 

 consists of a circular disc, whose outer edge is brighter than 

 the average and whose central part — that which corresponds to 

 the obstructed part of the object-glass — is blank ; but round the 

 central blank the intensity of photographic action is greater than 

 the average. To put it in other words, the mark is a broad ring, 

 whose external radius is twice the internal radius, and the in- 

 tensity at the edges of the ring, both external and internal, is 

 greater than the average within the broad ring; moreover the 

 brightness of the internal edge is greater than when no ob- 

 structing disc is used. The excess of brightness of the internal 

 edge is the point to which I wish to call attention. 



The second photograph was taken under similar circumstances 

 except that the obstructing circular disc was displaced from its 

 central position, and set so that its centre fell halfway between 

 the centre and edge of the full aperture. In this case also the 

 marks on the developed plate exhibit an intensification of the 

 light round the edge of what may be called the shadow of the 

 displaced disc. 



In both of the photographs described there were several ex- 

 posures of various durations on each plate. The effect of long 

 exposures is to obliterate the differences of intensity in different 

 parts of the marks. If the exposure is so short that the plate 

 is barely affected by the average intensity, then the parts where 

 the intensity is greater are seen with exaggerated distinctness. 

 It is easy to pick out amongst the various marks made with 

 different exposures, one or more that show the ring very 

 plainly. 



The features seen in the photographs suggest the idea that 

 the intensification of the light near the boundaries of these 

 star-marks is a result which is to be expected with an object- 

 glass of any shape of aperture, whether circular, square or tri- 

 angular. 



[Some photographs lately taken with a triangular aperture 

 show that this expectation is realized in fact. The triangular 

 marks are bounded by edges of which the intensity is greater 



