398 



NA TURE 



[Feb. 2 1, I ! 



(part 2).— The nervous system of the Holothuria pedata : 

 Cuvier's organ. The nervous system and sense organs of the 

 Holothuria apedata (plates 20-22). — On some new Species of 

 Thalassema, by Kurt Lampert, Erlangen. 



Vol. xxxix. Part 3, December 21, 1S83, contains: — On the 

 Rotifers of the environs of Giessen, by Karl Eckstein, nalural 

 history student, Giessen (plates 23-28), enumerates and describes 

 in detail fifty species (one new geiuis Distyla, with two new 

 species, D. ^iesseniis and £). hidwigii). A list of all known 

 genera is given, with a general description of the an.Ttomy, 

 development, and habits of the group. Avery complete biblio- 

 graphy is appended. — On the digestive apparatus of the 

 Decapods, by Dr. F. .\lbert (plates 29-31, and \voodcut>). 



Vol. xxxix. Part 3, December 3t, 1SS3, contain^ : — On 

 Bucephalus and Gasterostomum, by Dr. H. Ernst Ziegler 

 (plates 32 and 33) (Bucephalus polyniwrphus was found in con- 

 siderable quantities in Anodouta nititabilis). — On the central 

 nervous system in Periplaneta oriciitalis, by Dr. Max Koe^tler 

 (plate 34). — Ou the varieties of the cerebral fissures in Lepus, 

 Ovis, and Sus, by Dr. Victor Rogner (plate 35). — On the 

 structure and fissiparity of Cteiwdrilus monostyliis, sp. nov., by 

 Max Graf Zeppelin (plates 36and 37). — On the nervous system of 

 the snr>ut and n]^per lip in oxen, by Ivan B. Cybulsky (plates 38 

 and 39). — On the anatomy and physiology of the proboscis in 

 Musca, by Dr. Karl Krae|.elen (plates 40 and 41). — On the 

 connective tissue of the epiphyses in Plagiostomes, Ganoids, and 

 Teleoslea, by Dr. J. Th. Cattel. 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, J,anuary 10.— "On the Amount of Light 

 Reflected by Metallic Surfaces." By Sir John Conroy, Bart., 

 M.A. Communicated by Prof. Stokes, Sec.R.S. 



In a paper which Prof. Stoker did me the honour of commu- 

 nicating to the Royal Society, and which appeared in the Pro- 

 ceedings, vol. XXXV. p. 26, I gave an account of some experiments 

 I had made on the amount of light reflected by polished metallic 

 surfaces when ordinary unpol.arised light was incident upon them. 



The light of a ]iaraffin lamp fell either directly, or after reflec- 

 tion from the metallic surface, on a photometer, and the readings 

 were made by altering the distance at which another similar lamp 

 had to be pknced from the photometer in order to produce an 

 equal illumination. 



I have repeated the experiments with the steel and speculum 

 metal mirrors with polarised light. The polish of the tin and 

 silver mirrors being defective, it was not thought worth while to 

 re-examine them. 



The general arrangement of the apparatus remained the same ; 

 but in order to obtain a more intense light, a magic lantern (the 

 one known as the " Sciopticon" being used) was substituted for 

 the paraffin lamp carried by the goniometer. 



The metal plates were clamped to the vertical stage, and their 

 adjustment examined by pl.acing a second, or analysing, Nicol in 

 the path of the reflected light and crossing the Nicols, the former 

 being placed with its principal section either in or perpendicular 

 to the plane of incidence, and adjusting the stage screws till the 

 light reflected from the plate was completely extinguished. 



The experiments were made in the manner described in the 

 former paper, the light being polarised in, or perpendicularly to, 

 I he plane of incidence by the Nicol. It was found that the illu- 

 uination of the paper varied with the position of the Nicol, being 

 always greatest when the light which fell on the paper was 

 polarised in the plane of incidence. 



Four sets of observations and their means, made with the steel 

 and speculum metal mirrors, are given in the tables. 



Table I.— Steel, -.ml/t Light Polarised in the Plane of Incidence 



61-67 

 64-04 



67-41 

 70-41 

 74-55 



76-02 



80-77 

 84-92 



86-34 



63-06 

 68-18 

 71-97 



77-96 

 81-40 

 85-22 

 90-32 

 91-55 



61-05 

 62-90 

 69-41 



74-31 

 74-83 

 81-57 

 84-71 

 89-01 



60-70 

 64'2i 

 68-52 



74-42 

 77-37 

 82-26 

 86-01 

 87-87 



Table \Y\.— Speculum Metal, -with Light Polarised in the Pla 

 of Incidence 

 Angle of 



The amount of light which, according to Cauchy's theory, 

 ought to have been reflected by the mirrors was calculated out 

 by the formuk'e, the principal incidences and azimuths for the 

 two mirrors having been determined — 



, ., 9- -f cos- «- - 29 cos € cos I 



ff' + cos- i + 2fl cos e cos i 



6- cos- i + \ - 29 cos c cos i 

 B- cos- 2-1-1-26 cos f cos I 



and the observed and calculated results are set forth in Tables 

 V. and VI. 



Table V. — Amount of Li^ht Reflected by Steel Mirror 

 Observed Calculated 



60-70 

 64-21 

 68-52 

 74-42 

 77-37 

 82-26 



86-01 

 S7-87 



I- 

 50-19 

 46-28 



40-98 

 34-78 

 30-03 

 26-54 

 24-73 

 26-60 



63-17 



66-44 

 70-80 

 76-72 

 79-52 

 83-04 

 86-85 

 90-97 



r- 

 54-95 

 51-31 

 42-09 

 39-24 

 3532 

 31-62 

 29-46 

 32-39 



Table VI. — Amount of Light Reflected by Speculum Metal 

 Mirror 

 Observed Calculated 



As far as regards the general character of the phenomena the 

 agreement is complete and in accordance with the observations of 



