PHYSICS. 369 



soou as the exact relation is known of the liter to the cubic decimeter, 

 these values can of course be converted into centimeters. (J. Phys., 

 September, 188G, II, v, 411.) 



Pickering' has made an extended comi^arison of the photographed 

 normal diffraction spectrum, published in 1873, by the late Henry Dra- 

 per, and of Cornu's steel plate of the same region, with the photographs 

 of the solar spectrum recently issued by Powlaud. The results of the 

 measurements show that the maps differ systematically, the wave 

 lengths according to Draper's maps being too great for the lines of short 

 wave length. Applying the proper corrections to the two maps of 

 Draper and Cornu, the results agree closely with Kowland's. The 

 mean difference for the seventy-six lines compared was 0.012, corre- 

 sponding to about one eight-hundredth part of an inch upon the Draper 

 map. It may therefore be assumed that the probable error of a wave 

 length derived from the map of Draper will not exceed one one-hun- 

 dredth of a unit, if the correction given be first applied. The wave 

 lengths given on Cornu's map when thus corrected give an average 

 deviation of 0.025. (Am. J. Sci., September, 1886, III, xxxii, 223.) 



Cornu hasdevised a simple but very ingenious method for distinguish- 

 ing those lines in the sun's spectrum which are solar in their origin from 

 those which are terrestrial. Fizeauhad Shown that there is a displace- 

 ment of solar lines toward the red or the violet, according as the light 

 came from the receding or the advancing edge of the sun. No such 

 displacement occurs obviously if the lines are produced by absorption 

 in the earth's atmosphere. This displacement is very minute, being 

 only about yoo <^f the distance between the D lines as a maximum. But 

 this Cornu has found quite suflicient. Using a Rowland grating, an 

 image of the sun is formed on the slit of the spectroscope by a lens, to 

 which a slight oscillatory motion can be given by the hand. To dis- 

 tinguish between a line of solar and one of terrestrial origin the line is 

 brought near the vertical wire of the eye-piece, or better still, near one 

 of those inevitable grains of dust which are always seen on the horizontal 

 wire. The lever connected with the lens is then oscillated so as to bring 

 alternately the two ends of the solar equator tangentiallj^ upon the slit. 

 If the line is of terrestrial origin it remains absolutely fixed ; if it is solar 

 it oscillates with the lever. (Nature, July, 1886, xxxiv, 210; Phil. 

 Mag., November, 1886, V, xxii, 458.) 



Cornu at once applied this new method to the study of the solar spec- 

 trum groups a, B and A, and in an extended memoir has given a de- 

 tailed account of the modifications employed in the research. Fifteen 

 doublets were observed in the A group, thirteen in B, and eleven in a. 

 The paper is accompanied by an excellent plate giving all the lines of 

 the groups drawn to scale, and indicating those which are of solar and 

 those which are of terrestrial origin. These latter lines the author di- 

 vides into two classes, one of which is probably due to moisture. (Ann. 

 Chim. Phys., January, 1886, VI, vii, 5.) 

 H. Mis. 600 24 



