46

ELEMENTS OF MATERIA MEDICA.

Occasionally lenses of otlier forms, than those above enumerated, arc employed; but theonly one deserving of special notice is thc periscopic or meniscus (concavo-convcx) lens,recommcndcd by Dr. Woliaston (Nicholsons Journal, vols. vii. and viii.j for enlarging thefield of Vision. 1

c. C h r o m a t i c Instruments.

In sorne affections of the eye (popularly known as weukness of sight,) colouredglasses are employed, with occasional relief, to diminish the intensity of thelight. Those with a neutral tint (or twilight tinge) prove the most agreeable tothe eye.

White light is most fatiguing and hurtfu] to the eye. 2 The discase, ealled snow-blindness,which eometimes rcsults from the long couternplation of a country covered with snow, is pro-bably rctinilis. (Mackenzie, op. cit. p. 501.—Xenophon (Anabasis, lib. iv.) speaks ofsnow-blindness.) ßoth red and yellow light (Hence amber lenses are objectionabie) areinjurious to the eye. To the excess of the yellow and red rays, in common artifieial light,may be in part ascribed the baneful influence of this light in causing impaired Vision. Tvvomodes of preventing its ill effects have bcen suggested ; viz. the addition, by reflection, of theblue rays that are dcficient (as by the use of conical blue shades or reflectors around theHarne,) or the substraction, by absorption, of the red or yellow. rays that are in excess (as bypassing the light throngh some transparent medium of a blue tint.) 3 Green, blue,indigo, andriolct lights, are rnuch less injurious than either red or yellow. Spectacles of these colourshave been made for the use of those suffering with sensitive eyes, but they are inferior to theneutral tint before mcntioned, since after their removal from the eyes every object sometimespresents for a short period eomplirnentary tints; showing that these colours have fatigued theretinee. All dark-coloured glasses, however, and espccially black crape spectacles, are objec-tionabie, on aecount of their greater power of absorbing and radiating caloric, by which theyprove heating to thc eyes. 4

2. Calor. — Heat.

Physiological Effects. —All living beings, but especially the animals denomi-nated warm-blooded, generate heat. To all a certain temperature (which differsin different individuals) is essential to the maintenance of life; and hence caloricor heat is a vital Stimulus. (See foot-note at p. 44.) Increased beyond a certaindegree, it ceases to be vivifying: it may cause inflammation or apoplexy; it mayexhaust by its prolonged stimulant Operation; or, when its action is very violent,it may decompose the organized tissues by its chemical influence.

There are three modes of promoting or raising the temperature of warm-blooded animals, viz: —

1. The communication from without of sensible heat, either by the application of heated

substances to the body, or by the introduetion of radiant heat.

2. By augmenting the generation of heat wlthin the body; as by the use of stimulant foods

and drinks.

3. By diminishing the oooling influence of surrounding bodies; as by the use of clothing

made of substances which are bad conduetors of caloric.The effects of caloric communicated from without, on living beings, are three-fold, viz:—

1. Piiysical; including expansion or dilatalion, nndjlviditi/.

2. Chemical; comprising increased tendency to changes of composition and decomposition.

3. Dynamical, physiological, or vital; comprehending all changes in the condition of the

vital properlies produced by heat. These changes are of tvvo kinds:—«. Pritnary; excitement, or augmentation, of vital aetion.ß. Secondary; exhaustion, or diminution, of vital action.

i For Farther Information respecting spectacles. consnlt Mackenzie's Practica! Treatise on Diseases of theEye. 3d edit.. London, 1S40. pp 784 and 7112 ; Kitchener's Economy of the Eyes, Part I. (Spectacles,) London,1S24; and Cox's Spectacle Secrets, London. 1838.

2 The intense light caused hy the ignition of charcoal and the combustion of the metals effected hy theVoltaic Batteiy construeted hy Professor Groves, has produced on myself, as well as on some friends, lem-porary blindness. The Symptoms (which lusted two days in my case) vvere those of retinitfs, with profuselachrymation.

3 See Dr. James Hnnter's woik, Ott. the Influence of artifieial Light in causing Impaired Vision. Edin-burgh, 1840.

4 Melloni (op. supra cit ) ascertained the diatheruianous properties of coloured glasses to be as follows:—Of IU0 ineident rays, there arc transmitted by

Coloured glass. jtays transmitted.

Bright yellow...................... 34

Mineral greert ...................... 23

Coloured glass. Rays transmitted.

Deep violet ......................... 53

Vivid red ............................ 47

Clear blue ........................... 42

Vcry deep blue

19