PHYSICS. 367 



ring gave the percentage of the light reflected. A piece of optically- 

 worked black glass reflected .058 of the total incident light. Repolish- 

 iug in one case increased the light reflected from .04095 to .0445. (Na- 

 ture, November, 1886, xxxv, 64.) 



Howard Grubb has given a lecture at the Royal Institution npon tel- 

 escope objectives and mirrors, their preparation and testing, with par- 

 ticular reference to his own methods and results. (Nature, May, 1886, 

 XXXIV, 85.) 



Stroh has described, in a paper to the Royal Society, a lantern combi- 

 nation by which stereoscopic effects may be obtained on a screen, as in 

 ordinary projection. (Nature, May, 1886, xxxiv, 68.) 



3. Dispersion and color. 



The new spectrometer constructed by Hilger for the physical labora- 

 tory of University College, Dundee, has proved very satisfactory in its 

 performance, reading directly to one second of arc and giving reliable 

 results. Its construction is very simple. The collimator stands on a 

 heavy pillar by itself; and the circle, which is divided to five minutes 

 of arc on a ring 15 inches in diameter with six radial spokes, is carried 

 on another pillar. The telescope, counterpoised, turns on the same axis 

 but does not touch the circle at any point, and the reading is thus 

 managed : From the telescope-bearing a double girder with a semicir- 

 cular plate tied across its diameter with tubes of brass, stretches hori- 

 zontally above the semi-circumference of the divided circle. To the ends 

 of this girder are fixed two long- focus microscopes whose axes produced 

 intersect the divided circle at the extremities of a diameter. They are 

 read by means of a pointer and a spider-line micrometer whose head is 

 divided into three hundred parts, each of which represents one second 

 of arc. The microscopes are carried at such a height that they easily 

 pass the collimator, and they can be read in any position. The light 

 from the collimator passes entirely under the girder. (Nature, May, 

 1886, XXXI V, 92.) 



Cornu has described a method by which he has succeeded in obtain- 

 ing hydrogen spectrum tubes free from other substances. In the first 

 place it is necessary that the mercury pump should be as far as possible 

 from the apparatus which is to be traversed by the electric discharge, 

 and that the communication between them should be established by 

 many meters of helical tubes of glass, connecting with tubes of larger 

 diameter containing first, fragments of sulphur to arrest the mercury 

 vapor, and then copper turnings to take up the vapor of sulphur. All 

 the parts of the apparatus should be sealed to each other by fusion. 

 The hydrogen is produced by the electrolysis of dilute phosphoric acid, 

 in a V-shaped voltameter, one of whose limbs communicates with the 

 apparatus. The tubes are first exhausted, a few bubbles of gas are 

 admitted and the rarefaction is continued. The spectrum shows spec- 

 tra of carbon compounds. The battery current to the voltameter is 



