CAEE OF THE EYES. CAEE OF THE MICROSCOPE. 143 



be soon acquired ; and when it has once been formed, all difficulty 

 ceases. Those who find it unusually difficult to acquire this habit, 

 may do well to learn it in the first instance with the assistance of 

 the shade just described ; the employment of which will pei'mit 

 the second eye to be kept open without any confusion. — So much 

 advantage, however, is derived from the use of the Binocular 

 Microscope, even with objects not requiring its stereoscopic effect, 

 that the Author would strongly recommend its use to every observer 

 who wishes to take advantage of the best means of avoiding injury 

 to his sight. — There can be no doubt that the habitual use of the 

 Microscope for many hours together, especially by lamplight, and 

 with high magnifying powers, has a great tendency to injure the 

 sight. Every Microscopist who thus occupies himself, therefore, 

 wall do well, as he values his Eyes, not merely to adopt the various 

 precautionary measures already specified, but rigorously to keep to 

 the simple rule of not continuing to observe any longer than lie can 

 do so without fatigue.* 



108. Care of the Microscope. — Before the Microscope is brought 

 into use, the cleanliness and dryness of its glasses ought to be 

 ascertained. If dust or moisture should have settled on the Mirror, 

 this can be readily wiped off. If any spots should show themselves 

 on the field of view when it is illuminated by the mirror, these 

 are probably due to particles adherent to one of the lenses of the 

 Eye-piece ; and this may be determined by turning the eye-piece 

 round, which will cause the spots also to rotate, if their source lies 

 in it. It may very probably be sufficient to wipe the upper surface 

 of the eye-glass (by removing its cap), and the lower surface of the 

 field-glass ; .but if, after this has been done, the spots should still 

 present themselves, it will be necessary to unscrew the lenses from 

 their sockets, and to wipe their inner surfaces ; taking care to 

 screw them firmly into their places again, and not to confuse the 

 lenses of different eye-pieces. Sometimes the eye-glass is obscured 

 by dust of extreme fineness, which may be carried off by a smart 

 puff of breath ; the vapour which then remains upon the surface 

 being readily dissipated by rapidly moving the glass backwards and 

 forwards a few times through the air. And it is always desirable 

 to try this plan in the first instance ; since, however soft the sub- 

 stance with which the glasses are wiped, their polish is impaired in 



* The Author attributes to his rigorous observance of the above rule 

 his entire freedom from any injurious affection of his visual organs, 

 notwithstanding that of the whole amount of Microscopic study which 

 he has prosecuted for thirty years past, a large proportion has been 

 necessarily carried on by Artificial light, most of his diurnal hours being 

 occupied in other ways. He has found the length of time during 

 which he can " microscopize " without the sense of fatigue, to vary 

 greatly at different periods ; half-an-hour's work being sometimes 

 sufficient to induce it, whilst on other occasions none has been left 

 by three or four hours' almost continuous use of the instrument, — his 

 power of visual endurance being usually in relation to the vigour of his 

 general system. 



