December 22, 1892] 



NATURE 



191 



and tables of results given. Absorbing power was determined 

 by measuring the candle-power of a glow-lamp, first when un- 

 covered, and then when surrounded by a cylinder of the substance 

 under test. It was found to be of great importance to dis- 

 tinguish between apparent and real absorption, for reflection 

 from the surfaces of the cylinders increases the internal 

 illumination. The true absorption coefficient (a) is given 



by o = (i - rj) _5^m-'i, where ■t\ is the reflecting power and k-^ 



and X'lj the candle-powers with and without the envelope of 

 material under test. In determining transmitting power, the 

 Methven standa-'d and photometer were placed on one side of 

 the surface and the glow-lamp on the other. Difficulties were 

 experienced from the fact that some materials such as tracing 

 paper, transmit part of the light directly (like transparent sub- 

 stances), and another part by diffusions according to the cosine 

 law. Methods for discriminating between the different parts 

 were therefore devised both in the reflection and transmission 

 experiments, and consistent results subsequently obtained. 

 Tables and curves showing the close agreement of calculated 

 and observed values, are included in the paper. An abstract of 

 some of the tables of numbers is given below : — 



Theoretically the sum of the reflecting, absorbing and trans- 

 mitting powers should be unity, but from the above table it will 

 be noticed that they exceed lOO per cent., by amounts greater 

 than can be accounted for by experimental error. Th.s dis- 

 crepancy, the author thought, might be attributed to the law of 

 cosines not being exactly fulfilled. Mr. A. P. Trotter said he 

 had been interested in the subject of diffusion for many years 

 with a view to obviatmg the glare of arc lamps. Some experi- 

 ments he made on reflecting power gave unsatisfactory results, 

 owing, as he now saw, to his not taking the solid angles sub- 

 tended by the reflecting surfaces into account. The reflecting 

 power of substances was of great importance in the illumination 

 of rooms ; in one case measured by Dr. Sumpner and himself, 

 two-thirds of the total illumination was due to the walls. It 

 would greatly simplify measurement of reflecting power if some 

 substance could be adopted as a standard. Referring to the 

 cosine law, he said he had found it true, except when the angles 

 of incidence approached 90°. In cases where considerable total 

 reflection took place the apparent brightness near the normal 

 direction was greatly in excess of that in other directioas. 

 These points he illustrated by polar curves. He had also con- 

 idered what should be the nature of a roughened or grooved 

 surface to give the cosine law of diffusion. No simple geo- 

 metrical form of corrugations, &c., seemed to fulfil the required 

 conditions. Dr. Hofferr said the high numbers given for the 

 reflecting powers of substances were very interesting. Most 

 people had noticed the effect of laying a white table cloth in an 

 ordinary room. He had also observed that wall papers of the 



NO. 1208, VOL. 47] 



same pattern, but slightly different in colour, had very different 

 effects in producing increased illumination, and wished to know 

 if the influence of small differences in colour and texture on 

 diffusing power, had been investigated. Mr. Blakesley defended 

 the cosine law, and suggested that the summation of the powers 

 exceeding unity might be due to the fact that the enclosure 

 reflected heat as well as light, thus raising the temperature and 

 increasing the efficiency of the radiant. Mr. Addenbrooke said 

 the importance of the subject was impressed on him when he 

 passed through America three years ago and noticed the crude 

 manner in which electric lighting was there carried out. If 

 using good reflecting surfaces increased the illumination of a 

 room 50 per cent., it was like reducing the cost of electricity 

 from 8(/. to ^d. per unit. He could hardly conceive any subject 

 of more practical importance than the one before the meeting. 

 Dr. C. V, Burton did not understand why the cosine law should 

 be objected to, for it was possible that no surface was perfectly 

 diffusive. The effect of reflection from walls, &c., say in 

 illuminating a book would not, he thought, be so great as would 

 appear from the numbers given, for one usually read near a 

 light, and the reflected light falling on the book was only a 

 small part of the whole, on account of the greater distances of 

 the walls. Another member pointed out that in experiments 

 such as those described, it was very important to screen the 

 photometer and surfaces from all radiation other than that under 

 test. He rather doubted whether any surface reflected as well 

 as mirrors. White surfaces might appear to do so, but this was 

 probably because the eye would overestimate it, owing to the 

 superiority of white in aiding distinct vision. Dr. Sumpner in 

 reply said he had, as stated in the paper, used white blotting 

 paper as a standard of reflecting power and found it very con- 

 venient. His most careful measurements had been made on 

 whitish surfaces and not on coloured ones. Where one colour, 

 say red, preponderates in a room, the average light would be 

 much redder than that emitted by the source owing to the other 

 colours being absorbed. In considering illumination as related 

 to distinct vision, it was necessary to take account of the eye 

 itself, for the pupil contracted in strong lights and opened in 

 feeble ones. This subject he hoped to treat fully in a subsequent 

 gaper. 



Entomological Society, December 7. — Frederick DuCane 

 Godman, F.R.S., President, in the chair. — The President 

 announced the death, on December 2, of Mr. Henry T. 

 Stainton, F. R.S., an ex-President and ex-Secretary of the 

 Society. — .Mr. Jenner Weir exhibited a species of Acraea from 

 Sierra Leone, which Mr. Roland Trimen, F. R.S., who had 

 examined the specimen, considered to be a remarkable variety of 

 Telchmia encedon, Linn. It was a very close mi nic of Limnas 

 alcippus, the usual West African form of Limnas chrysippus. 

 The upper wings of the specimen were rufous and the lower 

 white, as in the model, and the resemblance in other respects 

 was heightened by the almost total suppression of the black 

 spots in the disc of the upper wings, characteristic of the usual 

 markings of T. encedon.— Mr. F. J. Hanbury exhibited a re- 

 markable variety of Lyccena adonis, caught in Kent this year, 

 with only one large spot on the under side of each upper wing, 

 and the spots on the lower wings entirely replaced by suffused 

 while patches. He also exhibited two specimens of Noctua 

 xanthographa of a remarkably pale brownish grey colour, ap- 

 proaching a dirty white, obtained in Essex, in 1891 ; and 

 a variety of Acronycta rumicis, also taken in Essex, with a dark 

 hind margin to the fore wings. — Mr. H. J. Elwes exhibited a 

 living specimen of a species of Conorephalus, a genus of Locus- 

 iida, several species of which, Mr. McLachlan stated, had been 

 found alive in hothouses in this country. — Dr, T, A, Chapman 

 exhibited immature specimens of Tccniocampa gracilis, T. 

 golhica, T. populeti, T. mtinda, T. instabilis and T. leucogra- 

 pha, which had been taken out of their cocoons in the autumn, 

 with the object of showing the then state of development of the 

 imagos. — Mr. F. W. Frohawk exhibited a living specimen of 

 the larva of Carterocephalus palamon {Hesperia paniscus)hyheT- 

 nating on a species of grass which he believed to be Bromus 

 asper. The Rev. Canon Fowler and Mr. H. Goss expressed 

 their interest at seeing the larva of this local species, the imagos 

 of which they had respectively collected in certain woods in 

 Lincolnshire and Northamptonshire. Mr. Goss stated that tht 

 food-plants of the species were supposed to be Plantago major 

 and Cynosurus cristatus, but that the larva might possibly Iced 

 on Bromus aiper. — Mr. C. G. Barrett exhibited a long series 

 of remarkable melanic varieties of Boarmia repandata, bred 



