246 Dr. Tyudall on the Progress of the Physical Sciences : 



distinct visiou the blue flame is lai'ger 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 ph?enomena of irradiation are connected by a chain of 

 experimental facts with chromatic phsenomena, which directly 

 point the way to the explanation of the former. The complete 

 explanation is embraced by the proposition, that for a given di- 

 stance the capacity of accommodation of the eye is different for 

 white 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 distances. Beyond the point of distinct vision, a micro- 

 meter drawn in black lines upon a white ground appears as a gray 

 spot, when dra^ni in white lines on a black ground it appears as a 

 bright one. If a series of parallel white lines be viewed through 

 a blue glass, the observer gradually receding until the lines run 

 into each other and are no longer distinct, from this distance the 

 hues, 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 considerably gi-eater 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 pig- 

 ments of such equal intensity that their combination shall ex- 

 hibit lustre, but the lustre can be reacUly 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 colom'cd glass held before both eyes. 

 ^Vith the ruby-glass and bright light the relief appears like 

 polished copper. In this way we learn that the results, as 

 regards lustre and irradiation, obtained with white and black, are 

 also true for any colours whatever. 



It is known that a green spot on a red field, which is moved 

 quickly hither and thither, apjiears to oscillate. Wheatstone 

 has showTi that a red heart on blue ground appears to oscillate 

 still more quickly; hence the appearance is not to be referred 

 to the action of complementary colours, but to a difference of 

 refrangibility. Sir David Brewster was the first to observe on 

 geological majis that blue and red do not appear in the same 

 plane, and the reason of this M. Dove considers to be rendered 

 completely evident by his stereoscopic experiments. His expla- 

 nation of the fluttering heart is as follows : — When the sheet is 

 moved in its own plane, the heart and the ground on which it 

 rests describe tangents of the same absolute length, but with 

 radii which the eye regards as different. The angular velocities 



