^86 REPORT— 1894. 



16. On the Correction of Optical Instruments for Individual Eyes. 

 By Tempest Anderson, M.D., B.Sc. 



The subject of astigmatism is now well understood, and the public are 

 beginning to recognise that a very large proportion of cases of defective sight 

 require a cylindrical as well as a spherical element in the corrective spectacles, 

 and can by this means obtain a much greater acuteness of vision than without it. 

 It is also well known that ordinary myopic or hypermetropic eyes, even if 

 presbyopic, can obtain perfect vision with a telescope or microscope by adjusting 

 the focus ; but it is quite dift'erent with astigmatic eyes, for as the image comes 

 into focus with one meridian of the eye, it goes out with the other. 



Persons with such eyes when ordered spectacles are often rather casually told 

 that they must wear their glasses when using a telescope or microscope, but there 

 the matter usually ends, for the spectacle glass is found to come against the eye- 

 piece, and even if it does not, the spectacle lens is not centred on the instrument, 

 and various ghosts and false images are formed, which cause the glasses to be 

 laid aside. 



Few eyes are altogether free from astigmatism ; probably half could have their 

 vision perceptibly improved by suitable cylindrical lenses ; and though the defect 

 is not sufficient to make it worth while to wear spectacles, it is greater than would 

 be allowed to exist in a telescope or microscope by a good maker. The defect is, 

 of course, more noticeable in the use of instruments which give a pencil of rays 

 which fully fills the pupil, such as the lower powers of telescopes and opera glasses 

 and hand-magnifiers. 



The remedy is simple, viz., to mount a cylindrical lens of power suitable 

 for the individual eye in a cap, which can be applied to the eye-piece of the 

 instrument. The lens can be properly centred, and causes no ghosts or false images, 

 and bemg close to the lens of the eye-piece takes up very little room, and allows the 

 eye to come convenienly near the instrument. Each observer will have one lens 

 to suit his own eyes, applied to his own instrument; but for the benefit of those 

 who like a complete instrument with adjustments to suit their friends, I may 

 point out that it is not necessary to correct the spherical error of the eye, as this 

 can be done by focussing, so that a set of caps containing, say, a dozen cylindrical 

 lenses of the ordinary powers would enable a telescope to be corrected for almost 

 any observer, provided he knew what cylinder he required. 



17. How the Misuse of the Word ^ Force,' in Attractions, Electricity, and 

 Magnetism, may he avoided tvithout much departure from existing 

 practice. By Dr. G. Johnstone Stoney, F.R.S. 



All the classical writers on dynamics of the early part of the present century — 

 Laplace, Lagrange, Poisson, Gauss, and many others — used the word Force in 

 two distinct senses, viz., for F the 'Moving force,' and for / the 'Accelerating 

 force.' A distinct improvement was introduced several years ago when the word 

 force was restricted to the former of these meanings, and when the ' accelerating 

 force' was named the acceleration, or, as it ought rather to be called, the 

 accelerator. 



There is, however, one exception. In treatises on Attractions it is still usual 

 to call / the force of attraction. There seems no sufficient excuse for this ; and it 

 is therefore suggested that the word accelerator be used instead. We may then 

 say in dealing, for instance, with gravitation — 



, M . ,, T-, ]\Im . ,, 



rnij -I /=-M-7, isthe F=-u— -isthe 



[Old names] •' '^ r- r^ 



Accelerating force Moving force 



Accelerator 

 [New names] (often miscalled ' Force of Forca 



attraction ') 



