144 SCIENTIFIC NOTES. 



Second — to use as few prisms as possible. Father Secchi,at Rome, 

 thinks he will be able to do very much with one prism and a very 

 long telescope. Grubb, of Dublin, for Lord Lindsay, uses one 

 prism and two halves, and sends the ray eight times through. 

 Hilger, of London, has made one with two prisms for Colonel 

 Campbell, and the ray sent four times through ; and the definition 

 is so perfect that nineteen lines have been ruled by the automatic 

 part of this instrument between the D lines. I believe the greatest 

 number ever recorded before was twelve, seen by Dr. Huggins with 

 the great Oxford spectroscope and bisulphide prisms. In a new 

 spectroscope that Hilger has made for the Sydney Observatory 

 there are three prisms of 64° each, so arranged that any power 

 may be obtained from two prisms to eighteen ; and the light is 

 sent six times through them. The definition of this instrument is 

 splendid. It was so far finished when I came away that I was 

 able to see the D lines in a small gas flame, perfectly sharp and 

 clear, through eighteen prisms ; and the success attending these 

 changes is so great that Mr. Hilger thinks he will be able to get 

 still better results by using one long prism and sending the ray 

 many times through it. 



While upon the subject of light, I may perhaps mention one or 

 two other matters. During the time I was in London, M. Cornu, 

 of Paris, exhibited at the Royal Institution and at the Physical 

 Society, his apparatus used in the new determination of the velocity 

 of light by Foucault's method ; the principal change is in using 

 telescopes to get the mirrors quickly and exactly into adjustment ; 

 and so great was the accuracy attained, that when the apparatus 

 was set upon the lecture table of the Physical Society, and the 

 reflected ray had only twenty feet to travel, the fact that it did 

 take time to travel even that short distance was manifest to all 

 who looked, myself amongst the number. As light travels twenty 

 feet in about the fifty millionth part of a second, this small interval 

 of time could be measured by this apparatus. Another curious 

 instrument was exhibited at the same meeting, viz., Mr. Crooke's 

 radiometer, based upon his discovery that any very delicately 

 balanced piece of pith or other substance, placed within a perfect 

 vacuum, begins to move if light is allowed to fall upon it. Having by 

 a number of experiments established the fact of motion under these 

 circumstances, his next care was to make the apparatus in such a 

 form that the motion might be continuous or revolving, and the 

 form adopted is as follows : — A small glass globe, about 2 J inches 

 in diameter, has within it a projecting stem of glass that reaches 

 nearly to the centre ; two straws forming a cross, with a needle 

 point through the centre and a little pith disc on each end, is 

 made to balance horizontally with the needle point in the little cup 

 in the end of the glass stem : the whole of the air is then pumped 

 out, so that the globe is very nearly, perhaps quite, a perfect 



