NATURAL PHILOSOPHY. 165 



of irradiation. They established the fact, that the deportment of 

 black and white towards the eye is exactly similar to that of two dif- 

 ferent colors. The lustre obtained by the combination of black and 

 white is peculiarly strong, so decided, indeed, that some, and among 

 others, the writer of this report, compared it to the lustre of lead 

 glance, or of tin, although the component white and black were both 

 perfectly dull and lustreless. According to the explanation already 

 given, one of these surfaces must appear in advance of the other, 

 the viewing of an object by the naked eyes by different degrees of 

 illumination with white light is analogous to those experiments with 

 colored light, where the object, to be distinctly seen, must be brought 

 nearer in the case of blue light than with red. A dark object will, 

 under the same conditions, appear further off than a white one, as 

 th,e red surface appears more distant than the blue. At the distance 

 of distinct vision, the flame of a candle, when viewed through the 

 violet glass, which permits the ends of the spectrum to pass and 

 extinguishes the middle, appears violet ; that is, the red flame is as 

 large as the blue. At the distance of distinct vision, a white ob- 

 ject also appears of the same size as a black one; at a greater 

 distance, the blue flame embraces the red ; that is, beyond the dis- 

 tance of distinct vision the blue flame is larger than the red one ; 

 and so also beyond this distance, the white object on black ground 

 appears larger than the black object on white ground. In this way 

 the phenomena of irradiation are connected by a chain of experimen- 

 tal facts with chromatic phenomena, which directly point the way to 

 the explanation of the former. The complete explanation is em- 

 braced by the proposition, that for a given distance the capacity of 

 accommodation of the eye is different for while and black. 



In a recent paper, M. Dove has added some proofs to those already 

 given, of the fact that blue and red are plainly visible at different dis- 

 tances. Beyond the point of distinct vision, a micrometer drawn in 

 black lines upon a white ground appears as a gray spot ; when drawn 

 in white lines on a black ground, it appears as a bright one. If a se- 

 ries of parallel white lines be viewed through a blue glass, the ob- 

 server gradually receding until the lines run into each other and are 

 no longer distinct, from this distance the lines, if observed through a 

 red glass, will appear quite distinct. The reader may in this way 

 easily satisfy himself that the distance of distinct vision is considera- 

 bly greater 'for red than for blue. In the same way it may be plainly 

 shown that the distance for white is also greater than for blue. It is 

 difficult to obtain pigments of such equal intensity that their combination 

 shall exhibit lustre, but the lustre can be readily obtained as follows : 

 A drawing in white lines upon a black ground is combined in the 

 stereoscope with another in black lines upon a white ground and 

 viewed through a colored glass held before both eyes. With 

 the ruby-glass and bright light, the relief appears like polished 

 copper. In this way we learn that the result, as regards lustre and 

 irradiation, obtained with white and black, are also true for any 

 colors whatever. 



