270 Dr. S. Bidwell. On Negative After-images, and 



Table I. 



Aperture Diameter. 



No. mm. Candle-metres. 



1 14-2 5600 



2 II -4 3600 



3 8-6 2050 



4 5 4 800 



5 4 440 



6 3 2 280 



7 2 4 160 



8 2 1 120 



Method II. — It is shown in fig. 6 how the colour-patch may be 

 viewed directly by means of a Huyghens' eyepiece. A diaphragm 

 having an aperture of 1 cm. is fixed in front of the prism (F, fig. 3) 

 and is seen in the eyepiece when properly placed as a sharply defined 

 bright disc illuminated by the coloured rays passing the slit-screen I. 

 The apparent diameter of the disc is about one-fourth of that of the 

 field of view. Its coloration is sensibly uniform, but the method cannot 

 be used to combine widely separated portions of the spectrum, and only 

 a single slit was generally opened. The white light, which in the pro- 

 duction of the pulsative after-image alternates with the coloured light, 

 passes through the iris-diaphragm P, and the lens Q to the silvered 

 mirror S ; thence it is reflected to the unsilvered mirror T of thin 

 plate glass, which directs some of the light upon the eyepiece V. For 

 most observations pieces of ground-glass were placed behind the iris- 

 diaphragm P and before the collimator-slit in order to subdue the 

 light. 



Method. III. — The apparatus is arranged as in fig. 3, but for the 

 white cardboard screen there is substituted a piece of ground-glass 

 covered with opaque paper, in which is cut a circular opening 1 cm. in 

 diameter, the colour-patch and the concentric white-light disc being 

 projected upon the opening. At a distance of 9 or 10 cm. behind the 

 glass is placed a Huyghens' eyepiece, its position being such that the 

 field of view is just filled with the coloured light. By the aid of this 

 device observations can be made much more satisfactorily than when 

 the image upon the ground-glass is viewed merely by the unassisted 

 eye. Rays from any part of the spectrum can be combined ; but the 

 absence of a surrounding white ground with which to compare the 

 colour of the pulsative after-image is often found to be inconvenient. 

 For some of the experiments a screen of thick brown paper attached 

 to the rocking arm of a metronome was arranged to eclipse the 

 spectrum rays periodically, without obstructing the white light ; thus 

 the pulsative image and the white light were seen in the eyepiece 

 alternately, each for a period of a little more than one second, and it 



