Feb, 1 6, 1888] 



NATURE 



383 



so small that it can be neglected, and that a spherical form is as 

 good if not better than any other ; and there is no doubt, for tele- 

 scopes of about this ratio, Sir John Herschel is quite right when 

 he makes the statement in "The Telescope, "p. 81, "that is a good 

 form that gives a good image .; and that the geometrical distinc- 

 tions between the parabola, sphere, and hyperbola, become 

 mere theoretical abstractions in the figuring and polishing of 

 specula." But in the case where the aperture of the mirror is 

 about one-sixth of the focal length the distinction betwen the 

 sphere and the parabola does exist and becomes a large quantity, 

 which only the Foucault method of working allows to be dealt 

 with properly. In enumerating the different plans used by opti- 

 cians in getting the parabolic curve, the author is in error in 

 stating that Lassell adopted the method of local polishing, as he 

 always used a large tool, and got the figure by alterations of 

 the stroke. Foucault was the inventor of the system of local 

 polishing, and this was afterwards used by Draper, who finally 

 rested on that as the best method of working. 



The author considers that when the focal length exceeds 40 feet 

 even with a theoretically perfect mirror the slightest touch or 

 variation in temperature will be sufficient to destroy good 

 definition with high powers, irrespective of the disturbing effects 

 of the atmosphere, and he comes to the remarkable conclusion 

 that "by decreasing the focal length the rays cross at a less 

 acute angle, and small variations in the reflecting surface have 

 not so detrimental an effect " — a statement that is entirely 

 unsupported. 



No actual tests of the work that the 18-inch mirrors will do are 

 given. The experiments on the thickness of silver-on-glass 

 films are interesting, as are also those on the effect of pressure 

 or heat in altering the colours or colour-bands seen between two 

 plane surfaces almost in contact. Dr. Draper, by actually 

 weighing the amount of silver deposited on a large surface, came 

 to the conclusion that it was about 1/200,000 of an inch thick ; 

 and the author, by comparing its thickness with the length of a 

 wave of light, comes to about the sam.e conclusion, and considers 

 that by ordinary care in polishing no optical change will be 

 produced in the reflecting surface by the film' of silver 

 deposited upon it. 



The roads to success in making the mirrors of a reflecting 

 telescope are many and various. Almost every maker in this 

 fascinating pursuit had his own that gave to him best results. 

 This was more particularly the case before Foucault published 

 his most admirable memoir on the construction of silver-on-glass 

 telescopes. In this memoir Foucault describes his method of local 

 polishing, and the tests that can be applied to the concave surface, 

 and a method of obtaining the true parabolic surface with 

 absolute certainty, bringing the art of specula-making at once to 

 a system of working by measurements in place of the old 

 empirical process that had up to that time been in use ; and 

 everyone now uses Foucault's method of testing concave surfaces, 

 and nearly everyone his plan of figuring by local polishing. 

 Mr. Madsen gives a very interesting account of the road he took, 

 an account that would have been much more valuable if the 

 details of the processes used in making both the concave and 

 the flat mirrors had been fully given, as it is now more in 

 the improvements in these details that gain is to be looked 

 for than in any of the main lines already known. 



A. AiNSLiE Common. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, February 2.— "On the Spectrum of the 

 Oxyhydr(^en Flame." By Profs. G. D. Liveing and J. 

 Dewar. 



In a former communication the authors described simultaneously 

 with Dr. Huggins the strongest portion of the spectrum of water ; 

 subsequently they described a second less strong but more re- 

 frangible section of the same spectrum. M. Deslandres has 

 noticed a third still more refrangible section. The authors now 

 find that the spectrum extends, with diminishing intensity, into 

 the visible region on the one hand, and far into the ultra-violet 

 on the other. These faint parts of the spectrum they have 

 photographed, using the dispersion of a single calcite prism and 

 a lengthened exposure ; and in the present communication they 

 give a map of the whole extent observed, and a list of wave- 

 lengths of upwards of 780 lines. 



The spectrum exhibits the appearance of a series of rhythmical 

 groups more or less overlapping one another, and the arrange- 

 ment of the lines in these groups is shown to follow, in many 

 cases, the law that the distances between the lines, as measured in 

 wave-lengths, are in an arithmetic progression. M. Deslandres 

 had previously announced that the succession of lines in several 

 spectra, as well as in the telluric groups A, B and a of the solar 

 spectrum, follow this law when their distances are measured in 

 reciprocals of wave-lengths, and he has stated that the groups 

 A, B and o have counterparts in the spectrum of water. 

 The authors find a striking resemblance between those groups 

 and certain parts of the water spectrum, but no exact corre- 

 spondence. 



Dr. Griinwald, of Prague, predicted on theoretical grounds 

 that certain lines would appear in the spectrum of water, and 

 the authors have found a considerable number of lines which tally 

 closely with Dr. Griinwald's predictions, some of them, in the 

 extremities of the spectrum, being the strongest lines observed 

 in those regions. 



February 9. — "True Teeth in the Young Ornithorhynchus 

 paradoxus." By Edward B. Poulton, M.A., F.L. S., of Keble 

 and Jesus Colleges, Oxford. Communicated by W. K. Parker, 

 F.P.S. 



This paper was a preliminary account of typical mammalian 

 teeth developing beneath the site of the horny plates, which sub- 

 serve mastication in the adult animal. In the upper jaw there 

 are three teeth on each side : in the lower jaw two teeth, corre- 

 sponding to the two posterior teeth of the upper jaw, were proved 

 to exist, but the anterior one may be also present, for the jaws ex- 

 amined were not complete. The animal in which the teeth were 

 found was about 8 "3 decimetres long in the curled up attitude in 

 which it had been received, and the larger hairs had alone ap- 

 peared above the skin. 



The anterior tooth of the upper jaw was long, narrow, and 

 simple, as compared with the others ; it was very fully developed, 

 containing completely formed dentine and enamel, and its apex 

 was nearly in contact with the lower surface of the oral epi- 

 thelium. All the other teeth were broad and large, those of 

 the upper jaw possessing two chief cusps on the inner side of 

 the crown, and three or four small cusps on the outer side, while 

 this arrangement was reversed in the lower jaw. Dentine was 

 only formed upon the large cusps, and was not present upon all 

 of these. The histological details and the manner of develop- 

 ment appear to be precisely as in the higher Mammalia, a fact 

 which strongly supports the identification of teeth with the 

 placoid scales of Elasmobranchs. If teeth are so extremely 

 ancient, then we should expect them to be unmodified in the 

 ancestral Mammalia, although the other more recently special- 

 ized characters in the higher mammals are found in a more 

 primitive condition in the former. 



The teeth were found in some sections of the skull prepared 

 for Dr. Parker by his son. Prof. W. Newton Parker. These 

 sections, which had not been examined by Dr. Parker, were 

 lent to the author, and Dr. Parker most generously encouraged 

 the publication of the discovery, and assisted the investigation 

 with other material. 



Mathematical Society, February 9. — Sir J. Cockle, F.R.S., 

 President, in the chair. — Messrs. A. E. H. Love and G. G. 

 Morrice were admitted into the Society. — The following com- 

 munications were made :— Further remarks on the theory of dis- 

 tributions, by Capt. Macmahon, R.A. — The free and forced 

 vibrations of an elastic spherical shell containing a given mass 

 of liquid, by A. E. H. Love. — On the volume generated by a 

 congruency of lines, by R. A. Roberts, — Isoscelians, by R. 

 Tucker. 



Edinburgh. 



Royal Society, January 16. — Prof. Chr3rstal, Vice-President, 

 in the chair. — Obituary notices of some former Vice-Presidents 

 of the Society were read. — Prof. Tait communicated a paper by 

 Prof. A. Macfarlane, on a problem in relationship. — Mr. W. 

 Peddie read a paper on transition-resistance and polarization at 

 platinum surfaces. He showed that transition-resistance in- 

 creases greatly while polarization is proceeding. The ratio of 

 the final to the initial resistance is in some cases as 2 to I, 

 when the electromotive force of polarization is equal to that of 

 a Daniell cell. From his results regarding the time-rate of in- 

 crease of polarization he deduced (lO"") cm. as the value of 

 the distance between the platinum and the layer of gas 



