158 MANAGEMENT OF THE MICROSCOPE. 



unless the use of the mirror be dispensed with, or some special 

 reason exist for placing it otherwise. If the object under exami- 

 nation be transparent, the lamp should be placed at a distance 

 from the eye about midway 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 necessary for other purposes. If 

 observations of a very delicate nature are being made, it is desi- 

 rable, alike by daylight and by lamp-light, to exclude all lateral 

 rays from the eye, as completely as possible ; and this may be 

 readily accomplished, by means of a shade attached to the eye- 

 piece of the microscope. Such a shade may be made most simply 

 of an oblong piece of card-board, having a circular hole cut in it, 

 by which it may fit upon the eye-piece or the upper part of the 

 body; its two ends should be turned up, so as to cut off all 

 lateral light ; its upper side should also be turned up, so as to 

 cut off the light from the front ; and a notch should be cut in 

 its lower edge, in the proper position to receive the nose. It 

 may be either painted black, or may be covered with black cloth 

 or velvet. 



77. Care of the Eyes. Although most Microscopists 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 un- 

 pleasant or injurious fatigue, than when he always employs the 

 same. Whether or not he do this, he will find it of great im- 

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

 This, to such as are unaccustomed to it, seems at first very em- 

 barrassing, 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 re- 

 stricting 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 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 permit the second 

 eye to be kept open without any confusion. The advantage of 

 the practice, in diminishing the fatigue of long- continued obser- 

 vation, is such, that no pains are ill bestowed by the Microscopist, 

 which are devoted to early habituation to it. There can be no 

 doubt that the habitual use of the Microscope for many hours 

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



