248 Di". Tyndall on the Progress of the Physical Sciences. 



of the vibrations causes the most perfect summation of impres- 

 sions, because the interferences which take place when the times 

 of oscillation are different then fall away. This uniformity 

 renders the tone pure, and, in the case of colours, renders them 

 homogeneous. Blue stands in the same relation to red that a 

 higher tone occupies with regard to a deeper one. With blue 

 the vibrations of the retina are more frequent than with red, as 

 the vibrations of the tympanum are moi'e frequent with a high 

 tone than with a deep one. Now it is proved that with deep 

 tones the limit of sensibility becomes contracted when the tones 

 become weaker ; and this is completely analogous to the case, 

 that by decreasing brightness, the limit of sensibility for the red 

 rays should become narrower. Hence with weak illumination, 

 red, as a colour, disappears ; while blue, on account of the greater 

 frequency of its vibrations, remains longer visible. 



" In this way," observes the Professor, " I explain to myself 

 the wonderful phsenomenon, regarding which, however, strange to 

 say, nobody has expressed wonder, that by the weak light of the 

 stars the blue of the firmament is rendered distinctly visible." 



Herewith is connected the fact, that a prismatic spectrum ob- 

 tained from light which has passed through a narrow aperture 

 has its colours towards the red end comparatively stronger when 

 the light is intense. This is peculiarly plain if the spectrum be 

 viewed through a dichromatic medium which permits the ends of 

 the spectrum to pass and extinguishes its middle, thus enabling 

 both ends to be immediately compared with each other. The 

 dark space beyond the red end of the spectrum, where the calo- 

 rific effect is a maximum, would probably become distinctly 

 visible if the intensity of the sunlight were considerably increased 

 by concentration. This would be the experiment of Savart ap- 

 plied to colours. Probably to the subject we are considering 

 belong the experiments of Sir David Brewster on the lines of 

 Fraunhofer in this portion of the spectrum ; although the facts 

 observed appear to be referred to the destruction of spherical 

 aberration, and not to the illuminating power of the telescope 

 apphed. In a similar manner the limits of action on an iodized 

 silver plate at the violet end of the spectrum become expanded 

 with increasing brightness. 



If a person pass suddenly from a brightly illuminated room 

 into a very dark one, and then appi'oach the place through which 

 the light enters until blue becomes distinct, it will be found that 

 red is at first much more vivid. The eye must remain for some 

 time in the darkened <oom before the retina becomes so sensitive 

 as in deep twilight. When this is attained, the person may re- 

 cede to a distance from the place where the light enters where 

 the blue is still distinctly visible, and find that the red has 



