ADDRESS. ] 7 



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 thickness of glass 

 traversed. The magnificent gratings of Rowland are a new power in 

 the hands of the spectroscopist, and as triumphs of mechanical 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 subject 

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

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

 fore 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, Toung and Forbes drew the con- 

 elusion that the velocity of light in vacuo varies from colour to colour, 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 repeated his experiments Avith special reference 

 to the point in question, aud has arrived at the conclusion that no varia- 

 tion exists comparable with that asserted by Young and Forbes. The 

 actual velocity differs little from that found from his first series of experi- 

 ments, and may be taken to be 299,800 kilometres per second. 



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

 vise 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 knowledge. In spite 

 of the admirable investigations of Plateau, it still remains a mystery why 

 soapy water stands almost alone among fluids as a material for bubbles. 

 The beautiful development of colour was long ago ascribed to the inter- 

 ference 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 index of the fluid. Some of the phenomena 

 -are however so curious, as to have led excellent observers like Brewster 

 to reject the theory of thin plates, and to assume the secretion of various 

 kinds of colouring matter. If the rim of a wine-glass be dipped in 

 soapy water, and then held in a vertical position, horizontal bands soon 

 begin to show at the top of the film, and extend themselves gradually, 

 •downwards. According to Brewster these bands are not formed by the 



1884. c 



