142 MANAGEMENT OF THE MICROSCOPE. 



such a manner that the light shall be at the left hand of the 

 observer. It is most important that no light should enter his eye, 

 save that which comes to it through the Microscope ; and the 

 access of direct light can scarcely be avoided, when he sits with 

 his face to the light. Of the two sides, it is more convenient to 

 have the light on the left ; first, because it is not interfered with 

 by the right hand, when this is employed in giving the requisite 

 direction to the mirror, or in adjusting the illuminating apparatus ; 

 and secondly, because, as most persons in using a Monocular Mi- 

 croscope employ the right eye rather than the left, the projection 

 of the nose serves to cut off those lateral rays which, when the 

 light comes from the right side, glance between the eye and the 

 eye-piece. The side-shades fitted by Mr. Collins to the eye-pieces 

 of his Harley Binocular (Fig. 39) may be advantageously employed 

 with every instrument of that class. — When Artificial light is em- 

 ployed, the same general precautions should be taken. The Lamp 

 should always be placed on the left side, unless the use of the 

 mirror be dispensed with, or some special reason exist for placing 

 it otherwise. If the object under examination be transparent, 

 the lamp should be placed at a distance from the eye about mid- 

 way between that of the stage and that of the mirror ; if, on the 

 other hand, the object be opaque, it should be at a distance about 

 midway between the eye and the stage ; so that its light may fall, 

 in the one case upon the Mirror, in the other case upon the Stage, 

 at an angle of about 45° with the axis of the Microscope. The 

 passage of direct rays from the flame to the eye should be guarded 

 against by the interposition of the lamp-shade ; and no more light 

 should be diffused through the apartment than is absolutely neces- 

 sary for other purposes. If observations of a very delicate nature 

 are being made, it is desirable, alike by Daylight and by Lamplight, 

 to exclude all lateral rays from the eye as completely as possible ; 

 and this may be readily accomplished 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. 



107. 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. This, to such as are unaccus- 

 tomed 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 



