134: THE MICROSCOPE AND ITS REVELATIONS. 



plished by means of a shade made like the upper part of a Mask, and 

 lined with black cloth or velvet, which should be fixed on the ocular end 

 of the Microscope. 



133. Care of the Eyes. Although most Microscopists who habitually 

 work with the Monocular microscope acquire a habit of employing only 

 one eye (generally the right), yet it will be decidedly advantageous to the 

 beginner that he should learn to use either eye indifferently; since by 

 employing and resting each alternately, he may work much longer 

 without incurring unpleasant or injurious fatigue, than when he always 

 employs the same. Whether or not he do this, he will find it of great 

 importance to acquire the habit of keeping open the unemployed eye. 

 Tins, to such as are unaccustomed to it, seems at first very embarrassing, 

 on account of the interference with the microscopic image, which is 

 occasioned by the picture of surrounding objects formed upon the retina 

 of the second eye; but the habit of restricting the attention to that 

 impression only which is received through the microscopic eye, may 

 generally be soon acquired; and when it has once been formed, all diffi- 

 culty 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 permit the second eye 

 to be kept open without any confusion. So much advantage, however, 

 is derived from the use of the Binocular arrangement, either stereoscopic 

 or non-stereoscopic, that the Author would strongly recommend its use 

 to every observer, save in cases of exceptional difficulty. There can be 

 no doubt that the habitual use of the Microscope, for many hours 

 together, especially by lamp-light, and with high magnifying powers, has 

 a great tendency to injure the sight. Every Microscopist who thus 

 occupies himself, therefore, will do well, as he values his eyes, not merely 

 to adopt the various precautionary measures already specified, but rigor- 

 ously to keep to the simple rule of not continuing to observe any longer 

 than he can do so without fatigue. 1 



134. 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 parti- 

 cles adherent to one of the lenses of the Eye-piece: and this maybe 

 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 



1 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 wh le amount of Microscopic study which he has prosecuted for forty-five 

 years past, a large proportion has been necessarily carried on by Artificial light, 

 most of his daylight hours having been 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 discomfort, 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 vigor of his general system. 



