348 



KNOWLEDGE & SCIENTIFIC NEWS. 



[February, 1906. 



Astronomical ObservaLtiorv SLnd 

 Spectacle Wearing. 



By S. L. Salzedo. 

 Many persons are no doubt under the impression (as 

 the writer was for many years) that the wearing of 

 spectacles is a complete and effective hindrance to the 

 use of a telescope. This may result in the loss to 

 astronomy of people who might develop into en- 

 thusiastic workers. In order to demonstrate the error 

 of this view it is sufficient to deal in turn with the vari- 

 ous defects of vision for which spectacles are 

 prescribed, and to show how they are overcome. 



The defects of vision which are corrected by means 

 of spectacles are four in number; they are : — 



I. INfyopia (short sight). 

 ■ 2. Hvpermetropia and presbyopia (long sight and 

 'old sight). 



3. Astigmatism. 



4. Muscular imbalance. 



In myopia, hvpermetropia, and presbyopia the only 

 purpose and effect of the lenses prescribed is to supple- 

 ment the defective focal length of the eye. The 

 telescope, however, is able to supplement the focus 

 of normal vision by a very large amount in either 

 direction, i.e., to shorten or lengthen the focus, thus 

 performing the verv same function as the glasses worn 

 before the eve. These defects may, therefore, be com- 

 pletelv disregarded. 



Of the remaining two visual defects, No. 4, that of 

 muscular imbalance, is a matter of defective co-ordina- 

 tion of the visual axes of the hco eyes, and is entirely in- 

 operative where one eye alone is used alternately. 



The fourth defect of the astigmatism is due to de- 

 fective sphericity of the eye, and is corrected by means 

 of a lens ground to form a section of a cylinder, either 

 in combination with any of the other lenses or alone. 



This defect is not susceptible of correction by the 

 optical structure of the telescope. It needs to be dealt 

 with specially'. As the writer is fortunate enough to 

 possess three of the above defects in combination 

 (hvpermetropia, muscular imbalance, and astigmatism), 

 he has been compelled to devise a method of correcting 

 the astigmatism likewise. This may be done very 

 simply : 



Having ascertained the power of the cylinder re- 

 quired, have an ordinary sunhead carefully fitted, in 

 place of the dark glass, with a cylindrical lens of the 

 required strength. The axis of the cylinder should be 

 indicated by a line on the outside of the cap for the 

 purpose of adjusting to the astigmatic axis of each 

 eye. In most cases the astigmatic axes of the two 

 eyes are different. It would, therefore, be preferable 

 to have two lines cut upon the outside of the cap in 

 such a way that when one line is vertical the axis of 

 the glass is adjusted, say, to the left eye (this line being 

 marked " L "), and when the other line marked " R " 

 is vertical the cylindrical axis is adjusted to the 

 astigmatism of the right eye. This cap may be screwed 

 on to the telescope like an ordinarv sunhead, or may be 

 fitted into the screw flange of the eye-piece and the 

 lines brought to the vertical as required. 



For this purpose it is best to give the optician the 

 oculist's prescription, cxplicitiv stating, however, that 

 it is only the cylindrical glass which is required.. 

 It is perfectlv evident, therefore, that no purely opti- 

 cal defect need stand in the way of the enjoyment by 

 all those who desire it of the many delightful hours 

 which may be yielded by the examination of the 

 heavens. 



Coast Denudation in 

 E^ngland. 



By Edward A. Martin, F.G.S. 



In 1867 the total area of England, excluding the fore- 

 shore, and the area covered by the tides, was 32,590,397 

 acres. In 1900 it had diminished to 32,549,019 acres. 

 In the intervening 33 years, England had diminished 

 in superficial area by 41,378 acres. TTiese are figures 

 which have been furnished by the Board of .Agriculture. 

 Taking an acre of land to be worth £2^, this would 

 imply a capital loss to the country of over ;^. 1,000,000. 

 The total denudation of the land between the years men- 

 tioned is in reality greater than shown, since, in the 

 return for 1900, are included the gains which have ac- 

 crued in those comparatively few coast-line areas where 

 the sea has actually receded. 



\ similar loss of land had, of course, been going on 

 for centuries prior to the period covered by the return 

 in question, and there are not many parts of the 



country where there do not exist local records and 

 traditions of former much greater extensions of the 

 land. 



A glance at a geological map of England is suffi- 

 cient to show at once why it is that some parts of the 

 coast have suffered more greatly from erosion than 

 others. Generally speaking, the more ancient the 

 rocks which form the coast, the more they are able to 

 withstand the denuding action of the waves. Thus 

 we have on the one extreme the iron-bound coast of 

 Cornwall, with its ancient palaozoic rocks, on which 

 the waves have but little effect, and on the other the 

 boulder-clay areas of the Norfolk coast, and of the 

 soulh-ea.stern portion of the Yorkshire coast. 



During the last two years efforts have been made 

 with a view to- call public attention to the loss by coast- 

 erosion, and if possible to obtain national .sympathy 

 lor the land-owners who have suffered. Individual 

 efforts at grovning and breakwater building are com- 

 paratively few, outside the areas of the larger coast- 



