476 



NATURE 



{Sept. 1 6, 1880 



prove to be suitable for the requirements of horticultural pro- 

 ducts of luxury aud high value, it may possibly be otherwise with 

 such productions. 



The above considerations obviously suggest the question : 

 What is the office of chlorophyll in the processes of vegetation? 

 Is it as has generally been assumed, confined to effecting, in 

 some way not yet clearly understood, carbon assimilation, and, 

 this done, its function ended ; or is it, as Pringsheim has recently 

 suggested, chiefly of avail in protecting the subjacent cells and 

 their contents from those rays of light which would be adverse 

 to the secondary processes which have been distinguished as 

 growth ? 



Appropriate as it would seem that I should attempt to lay 

 before you a rhumi of results bearing upon the points herein 

 involved, so numerous and so varied have been the investigations 

 which have been undertaken on the several branches of the 

 question in recent years, that adequately to discuss them Avould 

 occupy the whole time and space at my disposal. I must there- 

 fore be content thus to direct attention to the subject, aud pass 

 on to other points. 



(To be continued,') 



THE BRITISH ASSOCIATION 



REPORTS 



Report on the Tertiary Flora of the Basalts of the North oj 

 Ireland, by Mr. \V. H. Baily. — Described the plants of Miocene 

 deposits, consisting of variegated marls, resting on a leaf-bed 

 near Glenarm. Amongst the plants were Sequoia couttsiiz, S. 

 'lyelii, Fagtts dtiicaliutn, Nyssa ornithobyoma, Aralis broivnia, 

 Fraxiniis guillclnue. These and others have been drawn and 

 described. 



Report on the Viinparous Nature of the Ichthyosauri, by Prof. 

 H. C. Seeley. — Dr. Channen Pierce had formerly described a 

 specimen of Ichthyosaurus in the Museum at Bristol which 

 he considered contains a fcetus in the act of coming into the 

 world, which view is supported by Prof. Seeley, who showed, 

 reasoning by the analogy of the stomach of a crocodile, it was 

 impossible that this animal could have swallowed a smaller 

 ichthyosaurus, and its remains been retained in the stomach 

 in a perfect (form, and alludes to the spiral structure of the 

 coprolite, pointing to a small intestine, and thought it is im- 

 possible that the animal could have passed through them in 

 the process of digestion ; and alluded to the fact that all 

 German specimens show the head of the smaller projecting 

 towards the tail of the larger, though the reverse is the case in a 

 specimen at Madrid. But in Tubingen the most perfect speci- 

 mens occur, in which the smaller animals are found lying' com- 

 pletely preserved between the ribs of the parent animal ; though, 

 he suggests, in all cases viviparous characters may not have 

 obtained in all forms of ichthyosauri. 



77;!? Sixth Report of the Underground Water Connnittee was read 

 by Mr. De Ranee, who pointediout that the watershed separating 

 the basins of the Thames and Eastern Counties from those of the 

 Humber and the Severn also divides the area of heaviest rainfall 

 on the Paleozoic rocks, which are nearly all impermeable, from 

 those of Secondary age, receiving a rainfall of about 30 inches. 

 West of this line, with the exception of the Trias, no Secondary 

 rocks occur. In Lancashire, Cheshire, and the Midlands the 

 Triassic Sandstones absorb about one-third of the rainfall, giving 

 a daily average of 400,000 gallons to each square mile of country : 

 wells in these rooks are capable of drawing on several square 

 miles, and in suitable situations of yielding from 2 to 3 million 

 galbns per day. The discovery of the Manchester coalfield 

 beds at Winwick, near Warrirgton, under the New Red Sand- 

 stone, at a depth of only ^^40 feet, was described. He re- 

 ferred to the position of the New Red boring at Bootle, for the 

 Liverpool Corporation water supply, as very badly chosen, being 

 close to one of the existing wells. He then showed the gradual 

 attenuation in thickness of the Bunter Sandstones, in a southerly 

 direction, against the old Pala:ozoic axis, ranging from the 

 Belgian coalfield to the Mendips. 



Report on the Present Stale of our Knowledge of the Crustacea, 

 by C. Spence Bate, F.R.S.— This is Part v. of the Report, and 

 deals with the subjects of fecundation, respiration, and the green 

 gland. 



SECTION A. — Mathematical and Physical 

 Improved Apparatus for the Objective Estimation of Astig- 

 matism, by Tempest Anderson, M.D., B.Sc. — Astigmatism 

 h.as been defined as that condition of the eye in which refraction 

 is unequal in the different meridians. In order to obtain suitable 

 spectacles for correcting this defect, it is necessary to know 

 accurately the focal adjustment of the meridians of maximum 

 and minimum curvature, whence the focal lengths of glasses, 

 generally either cylindrical or cylindrical on one side and 

 spherical on the other, are readily calculated. Many plans have 

 been adopted for determining this ; some subjective, depending 

 on observations made by the eye itself, and generally using a 

 point of light or a series of radiating lines as an object. From 

 their appearances when viewed at different distances, and with 

 lenses of different powers, the focal adjustment of the different 

 meridians is at last obtained. 



The advantage of this group of methods is their theoretical 

 delicacy, as they work by judging of the perfection of certain 

 images refracted on the retina in a manner not very dissimilar 

 to that in which they are usually formed ; the practical disad- 

 vantage, that accurate observations are required from one who 

 has never been accustomed to make them. Hence objective 

 methods have been introduced. Their advantages are, substi- 

 tuting trained for untrained observation. Tlieir disadvantages — 



1. The vessels of the retina and the optic nerves, which are 

 mostly employed as objects, are seldom in exactly the position 

 desir.able for estimating the refi'action in different meridians, and 

 are often at a different distance from the optical system of the 

 eye from that at which the sensitive layer of the retina lies. 



2. They mostly require the optical defects, if any, and the 

 accommodation of the observing eye to be taken into account and 

 allowed for, thus introducing risk of error. 



In the author's two instruments, an image of a suitable object 

 thrown on the retina of the observed eye, is used as an object by 

 the observer, with the following advantages : — 



1. The patient's sensations may be entirely disregarded, or 

 only used as confirmatory. 



2. The image used is necessarily at the retina, and not before 

 or behind it. 



3. The accommodation or any defects in the refraction of the 

 observer's eye does not enter into the result, as the only function 

 of this eye is to observe the formation of the image on the 

 retina. 



In the first plan a lamp / is provided with a condensing 

 lens c, and a series of radiating wires w (supposed to be seen 

 edgeways in the figure), thus giving a bright field with black 

 lines on it. 



The whole slides on a graduated bar C, at the other end of 

 which is a convex lens y (4 and 10 dioptrics are the most con- 

 venient powers, i.e. 10 and 4 inch focus). Close to the lens, 

 and at an angle of 45° to its axis, is a plane mirror (ot), which 

 reflects the rays at right angles to their former path. The 

 instrument is to be held so that this pencil of rays enters [the 

 observed eye, and when the wire screen is at the proper distance, 

 an image of it is formed on the retina. The mirror has the 

 centre left unsilvered, as in an ordinary ophthalmoscope, and has 

 a disk of coirecting lenses behind it, to render the retina, 

 and the image on it, visible by the direct method. The ob- 

 served eye should have its accommodation relaxed by atropine. 



The bar is so graduated that when an image of the whole 

 or part of the screen is sharp on the retina, the figure opposite 

 the screen expresses the refractive error of the meridian by which 

 the image is produced. Hence if the image of the whole screen 

 is seen to be equally sharp, the eye is known to be not astigmatic, 

 and the gradu.;tion given the number of dioptrics by which it is 

 myopic or hypermetropic. If the lines be not all sharp at once, 

 then the most distant point at \\hich a distinct imnge of any 

 of the wires is formed on the retina gives the refractive error 

 of the meridian of minimum refraction (expressed in dioptrics), 

 and the point at which the line at right angles to this is best 

 defined gives that of the meridian of maximum refraction. The 

 least of these gives the spherical element of the correcting lens 

 required for distant objects, and the difference between the two 

 gives that of the cylindrical part. The meridian of maximum 

 refraction is that in which the line is visible when the wires are . 

 at the greatest distance. 



In the second plan the lamp, /, condensing lens, c, and wire 

 screen, %u, are similar, and only differ in size, the front lens, y, 

 and mirror, m, are also similar, but the lamp and -ii'ircs are 

 pernranently fixed by a tube, so that the wires are accuvatcly in 



