Sept. 1 6, 1 8 So] 



NATURE 



A77 



the principal focus of the front lens, y. By this means the rays 

 from the wires (or rather from the interval between them), after 

 refraction through the lens and reflection by the mirror, are 

 parallel. If received by an eye which is emmetropic, and with 

 its accommodation relaxed, an image of the wires is formed on 

 the retina. The light radiating from the image passes out 

 through the optical .system of the eye ; is rendered parallel and 

 able to form an accurate image on the retina of an emmetropic 

 eye observing through the hole in the mirror. 



If the observed eye be not emmetropic, it is only necessary to 

 introduce lenses of different powers close in front of it, so as to 

 correct the rays both entering and leaving the eye. If the 

 refraction be the same in all meridians, the image of all the 



wires is sharp with the same lens, and this lens is the one 

 required to correct the ametropia. If any astigmatism exists, 

 different lenses are required for rendering the images of the 

 different wires sharp. 



The strongest and weakest of these are the measures of the 

 errors of refraction of the two principal meridians, and the 

 difference of their numbers of dioptrics gives the cylindrical 

 element of the correcting glass required. 



In this form of apparatus a disc of correcting lenses behind 

 the mirror is not required, as the single correcting lens near 

 the observed eye corrects the rays both entering and leaving 

 the eye. 



For rapidly finding the proper lens a disk of lenses is used. 



liyopva. 



each a centimetre in diameter, and with intervals of one dioptric ; 

 a smaller disc is attached containing the quarter dioptrics, so 

 that by their combination intervals of one quarter of a dioptric 

 can be read — a degree of accuracy gieater than the estimation is 

 generally susceptible of. 



The proper lens being calculated, its spherical and cylindrical 

 elements are combined and put together before the eye. It it be 

 the correct one, all the lines are seen sharp at the same time ; 

 if not, further examination is made. 



The principal advantage of the first plan is that the adjustment, 

 being made by the motion of the wire screen, is continuous, and 

 correcting lenses are not required for measuring the refraction, 

 but only for rendering the retinal image visible ; its disadvauatge 

 that, as the rays are not parallel as they pass from the front lens, 

 past the mirror to the eye, it is necessary for the apparatus to be 

 very near, and at a determinate distance from the observed eye, 

 otherwise the readings of the scale are vitiated. This, however, 

 is not a serious objection. 



In the second plan the rays in the corresponding position are 

 parallel, and the instrument can be held at any convenient 

 distance, say one or two feet from the observed eye, and the 

 observer can get a view of the cornea at the same time as he 

 views the image, so that he can estimate the refraction at 

 different points of the cornea. 



It is hoped that this may eventually lead to the determination 

 of the refraction at different parts of conical cornea and other 

 eyes with irregular astigmatism, and the application of suitable 

 lenses to them. 



Since writing the above, I find mention of an instrument by 

 Coccius Stimmel, with an optical arrangement on the same 

 plan as my second, but I have not heard of its being in use in 

 this country. 



The makers are T. Cook and Sons, York. 



SECTION D.— Biology 



After Dr. Sclater hadread report of Socotra Committee, 

 Dr. J. B. Balfour gave a veiy interesting account of his visit 

 to the island. The zoological and botanical collections have not 

 yet been examined by specialists, but are expected to yield 

 results of great interest. As not nearly sufficient time had 

 been allowed for a complete exploration of the island, which 

 is almost entirely tmknown to Eitropeans, Dr. Balfour hoped 

 that another similar expeditionwould be organised ; but in that 

 case it should be earlier in the season than the last, which was 

 in the island during February and the early part of March. 

 During the latter part of the time the heat was too great for 

 it to be possible to do anything in the middle of the day. 

 Dr. Sclater mentioned that the ornithological collection indicated 

 distinct African affinities. 



Further Remarks on the MoUusca of the Mediterranean, by 

 J. Gwyn Jeflreys, LL.D., F.R.S. — This is a supplement to a 

 paper by the author, which was published in the Report of the 

 Association for 1S78. Since that time many of the species 

 which were supposed to be exclusively Mediterranean have been 

 discovered in the North Atlantic. The number of such Medi- 

 terranean species, which were given in the former paper, was 222. 



Bypemiciropvz. 



In the present paper 41 of those species are enumerated as 

 also Atlantic, thus reducing the number of exclusively Mediter- 

 ranean species to iSl, and it was remarked that the Atlantic 

 nudibranchs and Cephalopods had not been completely worked 

 out, these anounting -to 58, and being included in the above 

 residue of 181. 



On the Migration of Birds, and Messrs. Brown and Cor- 

 deaux's Method of obtaining Systematic Observations of the 

 same at Lighthouses and Lightships, by Prof. Newton, M.A., 

 F.R.S. (Abstract). — Citing a passage from an article by 

 the Duke of Argyll (Contemp. Ret:, July, 1880, p. l), the 

 author met with a direct denial the Duke's assertion that 

 of "the army of the birds" it might be said that "it cometh 

 not with observation," pointing out that all we know of [the 

 migration of birds arises from observation, and all we do not 

 know from the want of it, remarking also that if it were not 

 for observation we should not know that birds migrate at all, 

 and inquiring whether it is not by renewed observation alone 

 that we can hope to know more of their migratory movements. 

 The author then proceeded to describe briefly the nocturnal 

 passage of migratory birds as noticed by himself at Cambridge 

 for the past seventeen years, and urged the importance of simlar. 

 but more systematic observations being made at other statioins 

 Remarking upon the especial advantages of lighthouses and light- 

 ships for this purpose, he recounted the successful attem t made 

 in the autumn of 1879, with the sanction of the Triinty House 

 authorities and the Coinmissioners of Northern Lights, by Mr. 

 Harvie Brown and Mr. Cordeaux, to obtain a serits of observa- 

 tions from the lighthouses and lightships on the coast of Scot- 

 land and the east coast of England, the results of which were 

 embodied in a Report (noticed in Nature, vol. xxii. p. 25), and 

 showed that returns were obtained from nearly two-thu-ds of the 

 English stations, and as regards the Scottish stations, from about 

 two thirds on the west and one-half on the east coast, thus 

 proving the intelligent interest taken by the men employed in the 

 inquiry. This single Report naturally did not throw any new 

 liaht on the subject, but it would be contrary to all experience if 

 ateries of such reports would not, and he therefore strongly 

 urTed the Association to countenance the renewed attempts which 

 Messrs. Brown and Cordeaux were making, and to encourage 

 with its approval them and their fellow-workers, the men of the 

 li>^hthouses and lightships, who could best answer the_ question 

 whether knowledge of " the army of the birds " and its move- 

 ments "cometh not with observation." 



On Anthropolosrical Colour-Phenomena in Belgium and else- 

 where, by J. Beddoe, M.D., F.R.S. —In Germany, Switzerland, 

 and Belgium, through Governmental assistance, the colours of 

 the eyes and hair of all the children in the primary schools have 

 been observed and tabulated. The wTiter ib very desirous that 

 our own officials should lend similar assistance to the Anthropo- 

 metric Committee of this Association. The results hitherto 

 obtained have been of considerable importance, and those for 

 Beleium are well shown in the monograph and maps of Prof. 

 Vanderkindete. These bring out a remarkable contrast between 

 the Flemish and the WaUoon provinces of Belgium, and tend 

 strongly to prove the persistently hereditary character of 

 even such physical characters as the colour of the hair and 

 the iris. 



