PHYSICS. 4G5 



Haycraft Las described tbe model of a lens which he has used suc- 

 cessfully in his lectures. It consists of a piece of deal board cut in the 

 shape of the cross section of a double-convex lens, and mounted so as 

 to stand vertical. Four small squares of board are fixed on the two 

 sides of this model, near the toi> and bottom, so that they can move 

 about a center. To each of these a glass tube bent at an obtuse angle is 

 fastened, and strings passing through these tubes represent the visual 

 rays. {Nature, October, 1884, xxx, 543.) 



3. Dispersion and Color. 



Demar9ay has pointed out the advantage of employing in spectrum 

 work an induction coil having a shorter and larger secondary wire than 

 is usual. His coil gives a spark only about 5'"™ long, though it is 11.5*='" 

 in diameter and 23*^^ long. The primary and secondary wires are both 

 ^mm jjj diameter, but the latter is nearly three times as long as the 

 former and the coil has three times the usual condensing surface. The 

 spark, though short, is quite thick, its aureole being S'"'" in diameter. 

 For the examination of liquids the author uses a sort of wick made of 

 platinum wires twisted together and then rolled into a circle, with one 

 end rising out of the middle. This is placed in the small dish contain- 

 ing the solution to be examined, and the spark, which need not be 

 more than one fourth to one-half a millimeter, is passed to the upright 

 end from a positive electrode of large wire. (0. R., December, 1884, 

 xcix, 1022, 1069.) 



Thollou has published an illustrated monograph of the line, or rather 

 the group, D of the solar spectrum, as observed with his new compound 

 prism. He gives for comparison the group as observed by Huggins in 

 1863, by Campbell in 1865, by Russell in 1877, by Vogel in 1879, by 

 Gassiot in 1863, by Cooke in 1866, by Fieves in 1882, and by Cornu in 

 1884. The last two were obtained by means of a Rutherfurd grating. 

 The author's spectroscope separates the components proper of the line 

 D by 12 minutes. Between these extremes this instrument shows twelve 

 shari^ly defined lines, the wave-lengths of which are given. Of these 

 nine are telluric, j)roduced by some variable constituent of our atmos- 

 phere, moisture probably. Two are not telluric, the one belonging to 

 nickel, the other to an unknown element. The last seems to possess 

 the characteristics of both, and hence the author concludes must be 

 made up of two comiioneuts superposed, one telluric, the other metallic. 

 A drawing is given showing the appearance of this group, first, when the 

 eastern limb of the sun is on the slit, and, second, when the western 

 limb is so placed. The displacement of the solar lines and the non-dis- 

 placement of the telluric ones are marked. {J. Phys., January, 1884, II, 

 III, 5.) 



Cornu has published a valuable memoir on the group « of the solar 

 spectrum, having devised an exceedingly ingenious method of distin 

 guishing at a glance solar from telluric lines. On the slit of the col- 

 S. Mis. 33 30 



