THE REGENT PROGRESS OF PHYSICAL SCIENCE, 753 



appears to tremble, as the result of slight alternately opposite displace- 

 ments. But, if the seat of the absorption be in the atmosphere, it is a 

 matter of indifference from what part of the sun the light originally 

 proceeds, and the line maintains its position in spite of the oscillation 

 of the image upon the slit of the spectroscope. In this way Comu 

 was able to make a discrimination which can only otherwise be effected 

 by a difficult comparison of appearances under various solar altitudes. 



The instrumental weapon of investigation, the spectroscope itself, 

 has made important advances. On the theoretical side, we have for 

 our guidance the law that the optical power in gratings is proportional 

 to the total number of lines accurately ruled, without regard to the 

 degree of closeness, and in prisms that it is proportional to the thick- 

 ness of glass traversed. The magnificent gratings of Rowland are a 

 new power in the hands of the spectroscopist, and as triumphs of me- 

 chanical art seem to be little short of perfection. In our own report 

 for 1882, Mr. Mallock has described a machine, constructed by him, 

 for ruling large diffraction gratings, similar in some respects to that 

 of Rowland. 



The great optical constant, the velocity of light, has been the sub- 

 ject of three distinct investigations by Cornu, Michelson, and Forbes. 

 As may be supposed, the matter is of no ordinary difficulty, and it is 

 therefore not surprising that the agreement should be less decided 

 than could be wished. From their observations, which were made by 

 a modification of Fizeau's method of the toothed wheel, Young and 

 Forbes drew the conclusion that the velocity of light m vacuo varies 

 from color to color, to such an extent that the velocity of blue light 

 is nearly two per cent greater than that of red light. Such a variation 

 is quite opposed to existing theoretical notions, and could only be 

 accepted on the strongest evidence. Mr. Michelson, whose method 

 (that of Foucault) is well suited to bring into prominence a variation 

 of velocity with wave-length, informs me that he has recently re- 

 peated his experiments with special reference to the point in question, 

 and has arrived at the conclusion that no variation exists comparable 

 with that asserted by Young and Forbes. The actual velocity differs 

 little from that found from his first series of experiments, and may be 

 taken to be 299,800 kilometres per second. 



It is remarkable how many of the playthings of our childhood give 

 rise to questions of the deepest scientific interest. The top is or may 

 be understood, but a complete comprehension of the kite and of the 

 soap-bubble would carry us far beyond our present stage of knowl- 

 edge. In spite of the admirable investigations of Plateau, it* still re- 

 mains a mystery why soapy water stands almost alone among fluids 

 as a material for bubbles. The beautiful development of color was 

 long ago ascribed to the interference of light, called into play by the 

 gradual thinning of the film. In accordance with this view, the tint 

 is determined solely by the thickness of the film, and the refractive 



VOL. XXY. — 48 



