February i6, 1893] 



NATURE 



;8i 



careful analysis of the conlenis is prefixed to the memoir. Or. 



F. N. Cole (pp. 378-388) discusses the simple groups from 

 order 201 to order 500, and arrives at the conclusion that " the 

 possible orders of simple groups of compound order between 201 

 and 500 are reduced to 360 and 432." The volume closes with 

 a note (p. 389) by M. M. D'Ocagne, correcting a slight mistake 

 in a memoir by him in the 1888 volume, entitled " Sur certaines 

 courbes," and the title page and index. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, February 2. — "On a Meteoric Stone found 

 at Makariwa, near Invercargill, New Zealand." By G. H. 

 F. Ulrich, Professor of Mining and Mineralogy in the Uni- 

 versity of Dunedin, N.Z. Communicated by Prof. J. W, Judd, 

 F.R.S. 



The specimen described in this memoir was found in the year 

 1879 in a bed of clay, which was cut through in making a rail- 

 way at Invercargill, near the southern end of the Middle Island 

 of New Zealand. Originally, this meteorite appears to have 

 been about the size of a man's fist, and to have weighed four or 

 five pounds, but it was broken up, and only a few small fragments 

 have been preserved. The stone evidently consisted originally 

 of an intimate admixture of metallic matter (nickel-iron) and of 

 stony material, but much of the metallic portion has undergone 

 oxidation. Microscopic examination of thin sections shows that 

 the stony portion, which is beautifully chondritic in structure, 

 contains olivine, enstatite, a glass, and probably also magnetite ; 

 and through these stony materials the nickel-iron and troilite 

 are distributed. The specific gravity of portions of the stone 

 was found to vary between 3"3i and 3"54, owing to the unequal 

 distribution of the metallic particles. A partial chemical ex- 

 amination of this meteorite was made by the author and Mr. 

 James Allen, but the complete analysis has been undertaken by 

 Mr. L. Fletcher, F.R.S., of the British Museum. The analysis, 

 which when finished will be communicated to this Society, has 

 gone so far as to show that the percentage mineral composition 

 of the Makariwa meteorite may be expressed approximately by 

 the following numbers : nickel-iron i, oxides of nickel and iron 

 10, troilite 6, enstatite 39, olivine 44. 



Physical Society, January 27, — Walter Baily, Vice-Presi- 

 dent, in the chair.— Prof. S. P. Thompson, F.R.S., made a 

 communication on Japanese magic mirrors, and exhibited 

 numerous specimens showing the magic properties. Referring 

 to the theory of the subject, he said the one now generally 

 accepted was that proved by Profs. Ayrton and Perry in 1878, 

 who showed that the patterns seen on the screen were due to 

 differences in curvature of the surface. The experiments he now 

 brought forward fully confirmed their views. Brewster had 

 maintained that the effects were due to differences of texture in 

 the surfaces causing differences in absorption or polarisation, but 

 the fact that the character of the reflected image depended on 

 the convergency or divergency of the light, and on the position 

 of the screen, showed this view to be untenable. Another proof 

 of the differing curvature theory was then given by covering a 

 Japanese mirror with a card having a small hole in it. On 

 moving the card about, the disc of light reflected from the ex- 

 posed portion varied in size, showing that the curvatures of por- 

 tions of the surfaces were not the same. The same fact was 

 proved by a small spherometer, and also by reflecting the light 

 passing through a coarse grating from the mirror, the lines being 

 shown distorted. To put the matter to a test demanded by 

 Brewster, he had a cast taken from a mirror by his assistant, 

 Mr. Rousseau ; this had been metallised, silvered, and polished, 

 and now gave unmistakable evidence of the pattern reflected 

 from the original. The true explanation of how the inequalities 

 of curvature were brought about during manufacture had also 

 been given by Profs. Ayrton and Perry, but there were some 

 questions of detail on which difference of opinion might exist. 

 The late Prof. Govi had noticed that warming a mirror altered its 

 possibilities. A thick mirror which gave no pattern whilst cold 

 developed one on being heated, was shown to the meeting. 

 Prof. Thompson also showed that a glass mirror having a pattern 

 cut on the back developed magic properties when the mirror 

 was bent. When made convex the reflected pattern was dark 

 on a light ground, and when made concave, light on a dark 

 ground. Warming ordinary mirror-glass by a heater whose sur- 

 face was cut to a pattern gave similar effects. Very thick 



glasses could be affected in this way. On passing a spirit lamp' 

 behind a strip of mirror, a dark band could be caused to pass 

 along the screen illuminated by light reflected from the mirror. 

 By writing on lead foil and pressing the foil against a glass 

 mirror by a heater, the writing was caused to appear on the 

 screen. Prof. Thompson had also found that Japanese mirrors 

 which are not "magic" when imported, could be made so by 

 bending them mechanically so as to make them more convex. 

 In conclusion, he showed a large mirror 15" x 11", the reflec- 

 tion from which showed the prominent parts of the pattern on 

 its back with the exception of two conspicuous knobs ; these 

 knobs gave no indication of their existence. Prof. Ayrton said 

 the simple mechanical production of the magic property 

 described by Prof. Thompson led him to think that some 

 experiments on " seeing by electricity " by the aid of selenium 

 cells which Prof, Perry and himself made some years ago, might 

 lead to some result if repeated with thinner reflectors. Speak- 

 ing of the effect of scratching the back of a Japanese mirror, he 

 pointed out that if metal be removed by pressure a bright 

 image was seen, whilst if removed chemically a dark image 

 resulted. Since the original paper on the subject was 

 written he had been led to modify his views as to the 

 effect of amalgamation, for some time ago he showed the 

 society how brass bars were bent if one edge be amalgamated, 

 thus proving that enormous forces were developed. He now 

 regarded amalgamation as an important part of the manufacture. 

 Mr. Trotter inquired if it had been proved that there was no 

 difference in the metal in the thick and thin parts ? One would 

 expect the thin parts to be harder and polished away less. 

 After some remarks by Mr. J. W. Keartonand Major Rawson, 

 Prof. Thompson said the magic effects produced by heating 

 the back of a glass mirror remained for a short time after the 

 heater was removed. The question of whether differences in 

 hardness of the thick and thin parts of a mirror were of 

 consequence in the production of the magic property had beea 

 tested by using sheets of brass thickened by pieces soldered to 

 the back as mirrors, and found to be unimportant. Prof. 

 Ayrton also described an experiment pointing to the same con- 

 clusion. — Mr. W. F. Stanley read a paper on the functions of 

 the retina — (i.) The Perception of Colour. Referring to 

 Young's three-nerve theory of colour-sensation, the author said 

 Prof. Rutherford had pointed out that there was no necessity to 

 assume that different nerves conveyed different colour-sensa- 

 tions, for as a telephone wire would transmit almost an infinite 

 variety of sound vibrations, so the nerves of the retina were 

 probably equally capable of conveying all kinds of light vibra- 

 tions. Prof Rutherford had further pointed out that the image 

 of a star could not possibly cover three nerve-terminals at once, 

 and therefore could not be seen as white if Young's theory was 

 correct. The author then described Helmholtz's experiments 

 with a small hole in a screen illuminated by spectrum colours. 

 For red illumination the greatest distance at which the hole 

 could be seen sharply defined was 8 feet, and for violet i\ feet. 

 When the hole was covered with purple glass, or with red and 

 violet glasses superposed, and a bright light placed behind, the 

 eye, when accommodated for red light, saw a red spot with a 

 violet halo round it, and when focussed for violet light, saw a 

 violet spot with circle of red. These experiments the author 

 thinks show that the chromatic sense in distinct vision under 

 critical conditions {i.e. where a single nerve or a small group of 

 nerves is concerned) depends on the colours being brought to 

 foci at different distances behind the crystalline lens. He also 

 infers that the same focal position in the eye cannot convey 

 simultaneously the compound impression of widely separated 

 colours. Helmholtz's observations are further examined in the 

 paper, and a series of zoetrope and colour disc experiments de- 

 scribed which tend to show that the eye cannot follow rapid 

 changes of colour. Changes from red to violet could be followed 

 much more quickly than from violet to red. The red im- 

 pressions were, however, more permanent. The observed 

 effects were found to depend on the intensity of the light, and 

 also on the distance of the eye from the coloured surface. 

 Summing up his observations, the author infers that by systems 

 of accommodation of the eye, the colours of the spectrum are 

 brought to focus on special parts or points of the 

 rods or cones of the retina, such focal points being 

 equivalent, by equal depths or distances from the crystalline lens, 

 to a focal plane formed across the whole series of nerve terminals. 

 That all the rays of light from an object, or part of an object, 

 of very small area and of any spectrum colour, will converge to- 



NO. 12 16, VOL. 47] 



