138 



SCIENTIFIC NE^VS. 



[Feb. lo, i8 



of papery!, Hectucc^f, etu 



THE ROYAL INSTITUTION. 

 On the 21st ult., Lord Raleigh commenced a course of 

 lectures on experimental optics, illustrated by electric 

 light. Beginning with the consideration of the enfeeble- 

 meat of light by distance it was pointed out that this 

 phenomenon quite harmonised with Sir Isaac Newton's 

 theory, that actual material corpuscles emanated from 

 the source of light and spread in all directions as they 

 travelled. But it also harmonised with Thomas Young's 

 wave or undulatory theory. It was most remarkable to 

 find two theories so totally distinct, each equally sup- 

 ported by the fact of the enfeeblement of light by dis- 

 tance. But when the matter was more closely looked 

 into, it was seen that the coincidence was near. Both 

 assumed a something travelling outwards and spreading, 

 corpuscles in one case and waves in the other — -actual 

 matter in the former, energy in the latter. In both 

 cases the enfeeblement would be inversely as the square 

 of the distance travelled. On this law were based the 

 various methods of photometry now practically em- 

 ployed. Count Rumford's now historical method was 

 selected as an illustration of the comparison of two 

 lights, in this case an electric glow light as against a gas 

 jet. The two lights threw a shadow of the same rod on 

 to a screen, the intensity of the shadows varying with 

 the distances of the lights. When the two shadows 

 were equal the distance was ascertained in inches, and 

 the intensity of the two lights thus compared. In 

 this case the ratios worked out — gas, 11 -025 ; glow light, 

 1 6 -900. When two lights were very disproportionate it 

 was, however, more difficult to compare them, and it could 

 not be done with the same accuracy. One form of 

 photometer shown was a revolving perforated disc. The 

 area of perforations and the intensity of the standard 

 were known, and the speed of rotation was measured. 

 When the shadow of a rod from a light to be tested was 

 matched by the shadow from the intermittent light the 

 value of the tested light was at once known. The next 

 subject illustrated was the reflection of light. From 

 such a surface as a white screen the light appeared to be 

 given off independently of how it fell on it. But when a 

 powerful contracted beam was reflected from a highly- 

 polished surface, held obliquely, it was seen that the 

 angle at which it left the mirror was the same as that 

 at which it fell on it. The electric light and a silver 

 mirror such as was used for astronomical purposes 

 showed this with remarkable definition on a large 

 scale. The subject of the " perversion " of a looking- 

 glass, changing right to left, was next considered. Letters 

 or figures written on some transparent substance, and 

 placed in front of a looking-glass, helped to realise that 

 what was seen by reflection was what would be seen 

 directly on the other side of the glass. The amount of 

 light reflected by mirrors was an important consideration. 

 In the astronomical silver mirrors now in use it was 

 about 95 per cent., with speculum metal about 60, and 

 bright steel far less. 



ROYAL ASTRONOMICAL SOCIETY. 

 At the meeting held on the 13th ult., the Rev. Father 

 Perry read a paper on the Condition of the Solar Surface 

 during the year 1 88 7, as observed at Stonyhurst College 



Observatory. The sun was observed on eleven days in 

 January, eighteen in February, twenty-four in March, 

 twenty-two in April, twenty-two in May, twenty-eight in .. 

 June, thirty in July, twenty-five in August, twenty-one 

 in September, twenty-three in October, seventeen in 

 November, and eighteen in December, making a total of 

 259 days. 



Mr. E. J. Stone read a paper entitled " Observations 

 of the Moon made at the Radcliffe Observatory, Oxford, 

 during the year 1887, and a Comparison of the Results, 

 with the Tabular Places given by Hansen's Lunar 

 Tables," in which he pointed out that the difference 

 between the observed places and the tabular places had 

 been gradually increasing since 1864, and now amounted 

 to as much as fifteen seconds and eight-tenths. Mr. 

 Stone contended that such a divergence could not be 

 neglected or passed over by altering the constants 

 of the lunar theory. Had a change in the theory been 

 made, and new tables made use of, Neptune would never 

 have been discovered. 



Mr. Bryant laid before the meeting a paper on the 

 " Opposition of Sappho," containing an ephemeris for the 

 opposition which takes place next March and April, with 

 a list of comparison stars, of which he asked observers 

 to make use. 



The Astronomer Royal, in a paper on the Spectro- 

 scopic Observations of the Motion of the Stars in the Line 

 of Sight taken in the year 1887, said there were two 

 points which he thought might be of interest. Sirius 

 had shown a complete reversion of its motion during the 

 time of observation at Greenwich, and since Dr. Huggins's 

 first results. In 1868 Dr. Huggins observed the motion 

 away from us as +15 miles per second. In 1876 the 

 motion was 21 miles per second away. In subsequent 

 years the following results were obtained -|- signifying 

 motion away from, and - motion towards the earth : — 

 1887-^12, i878-f22, 1880-1-16, I88I-^4|, 1882+4, 

 1883-4, 1884-21, 1885-20, 1886-20, 1887 + 6.' 

 Last year, therefore, we had come to a small plus motion 

 again. These results must, of course, be accepted with 

 some caution, as the line in the spectrum observed in 

 these observations was always the F line, which was a 

 very broad line in the spectrum of Sirius, and that 

 line had somewhat changed in its character, so that 

 it might be that there had been some change of the 

 absorption which might have affected the place of the 

 line in the spectrum of Sirius, so that the observations 

 really might not indicate a change of radial motion. 



Mr. Turner read a paper by Mr. J. R. Hind on " The , 

 Path of the Total Solar Eclipse of January ist, 1889." , 

 The only two eclipses worth observing for the next two 

 or three years are this one and one in December of the 

 same year, which is visible at a place on the west coast 

 of South Africa, about fifty miles south of St. Paulo de 

 Loanda. The eclipse there is of rather long duration — 

 about 3I minutes — and it may be found a very favour- 

 able opportunity for observing. 



ROYAL BOTANIC SOCIETY. 

 At a meeting of this Society, held on January 28th, Mr. 

 G. P. Gassiott (Vice-President) in the chair, it was 

 announced that the donations included seeds of the sweet- 

 scented Australian gum tree, Eiica/yplits citriodora, pre- 

 sented by the Hon. A. Deakin, of Melbourne. This is 

 one of the less frequent of its kind, being found only in 

 a few localities in New South Wales and Queensland. 



