346 On the Colours of Mixed Plates. 



parallel beam of light from an optical lantern), this expansion 

 of the rings with increasing obliquity of observation may be- 

 followed np till the direction in which the film is seen makes 

 an angle up to 90° with the direction of the incident light. 

 It is a noteworthy fact, that a film too thick to show colours 

 when observed nearly in the direction of the transmitted 

 light will show the coloured rings vividly when viewed at a 

 moderate obliquity. Ultimately all films, whether thick or 

 thin, appear practically achromatic in a sufficiently oblique 

 direction. The thinner the film, the smaller the angle of 

 diffraction necessary for this. The thinnest films, which 

 may be obtained by forcing the glasses together till they 

 nearly come into contact, scatter much less light than the 

 thicker films. Hence the film which is achromatic when 

 viewed obliquely shows a darker area near its centre. 



Observations in Monochromatic Light : JSormal Incidence. 



In this case, if the mixed plate is held directly in the line 

 of sight between the eye and the source of light at a suffi- 

 ciently great distance from both of them, a series of perfectly 

 black rings (alternating with bright rings) may be seen on 

 the film, even if this be fairly thick. Bringing the film 

 nearer the source of light or increasing the dimensions of 

 the latter has a very deleterious effect on the perfect blackness 

 and sharpness of the rings. Placing the eye a little out of 

 the direct line also results in the rings becoming blurred in 

 appearance; and, indeed, on merely bringing the film nearer 

 the observer in the line of sight so as to increase the angle it 

 subtends at the eye, the rings may appear broken up and 

 blurred in parts. (The effect is the more striking the thicker 

 the film and the hirger its area.) If a particular dark ring 

 be watched as the eye is gradually moved out of the line 

 through film and source, it, will • be noticed that the ring 

 becomes blurred and broadened, then again sharp and per- 

 fectly black, but with its position shifted outwards in the 

 film ; this process further repeats itself, but with rapidly 

 diminishing fluctuations in the intensity and sharpness of the 

 ring. Viewed at a moderately large obliquity, the dark 

 rings are always perfectly black, and their sharpness is much 

 less dependent upon the use of a light-source of restricted 

 area. The rings expand with increasing obliquity of obser- 

 vation and move out ^f the film, until finally no rings are- 

 visible at all. A darker patch at the centre, where the film 

 is thinnest, can be seen, as in the case of white light. 



