NATURAL PHILOSOPHY. 167 



deep voice of the seaman, strengthened by the speaking trumpet, is 

 lost in the storm, the shrill pipe of the boatswain still pierces through 

 the howl of winds and roar of waves. Savart has shown, by means of 

 the toothed-wheel, that the limit of sensibility of the ear for grave 

 tones is extended by strengthening the strokes. The complete simi- 

 larity of the vibration 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 colors, renders them homogeneous. Blue 

 stands in same relation to red that a higher tone occupies with regard 

 to a deeper one. With blue the vibrations of the retina are more fre- 

 quent than with red, as the vibrations of the tympanum are more fre- 

 quent 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 

 color, 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 phenomena, regarding which, however, strange to say, no- 

 body 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 obtained 

 from light which has passed through a narrow aperture, has its colors 

 towards the red end comparatively stronger when the light is intense. 

 This is peculiarly plain if the spectrum be viewed through a dichro- 

 matic 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 calorific effect is a maximum, would probably be 

 distinctly visible if the intensity of the sunlight were considerably in- 

 creased by concentration. This would be the experiment of Savart 

 applied to colors. 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 illu- 

 minating power of the telescope applied. 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 approach 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 dark- 

 ened room, before the retina becomes as sensitive as in deep twilight. 

 When this is attained, the person may recede to a distance from the 

 place where the light enters, where the blue is still distinctly visible, 

 and find that the red has vanished completely. Another remarkable 

 fact observed by M. Dove was, that among the numbers to whom he 



