MANAGEMENT OF THE MICROSCOPE. 133 



nating beam, when very high powers are employed, by mere hand-shift- 

 ings of the lamp and its condenser, Messrs. Dallinger and Drysdale, in 

 the admirable investigations of whose results a summary will be given 

 hereafter (Chap, xi.), have found great advantage from the use of a 

 Lamp mounted on a base to which a traversing horizontal movement can 

 be given in any direction by rectangular screws, and furnished with an 

 upright standard carrying two racks, on which the lamp itself and the 

 bull's-eye condenser can be separately raised or lowered by milled-head 

 pinions. By this more exact method of adjustment, the observer is able, 

 after a little experience in its use, to secure that most perfect position of 

 the flame and condenser, which Ordinary hand-adjustment might not suc- 

 ceed in attaining until after a great expenditure of time and patience. 1 



132. Position of the Light. When the Microscope is used by day- 

 light, it will usually be found most convenient to place it in such a man- 

 ner 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 in- 

 terfered with by the right hand, when this is employed in giving the 

 requisite direction to the mirrr, or in adjusting the illuminating appara- 

 tus; and, secondly, -because, as most persons in using a Monocular Micro- 

 scope 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. 49) 

 may be advantageously employed with every instrument of that class. 

 When Artificial light is employed, the same general precautions should be 

 taken. The Lamp should always be placed on the left side, unless some 

 special reason exist for placing it otherwise; and if the Object under ex- 

 amination 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. 

 In the examination of objects of the greatest delicacy and difficulty, 

 however, in which it is important to get rid of the reflection from the 

 front surface of the Mirror, a rectangular Prism should be substituted for 

 it, when the conditions of the observation necessitate the use of the 

 Microscope in the vertical position; but when the instrument can be 

 inclined, the Lamp may be most advantageously placed in the axis of the 

 Achromatic Condenser or other Illuminator, so that its light may be 

 transmitted to the object without intermediate reflection. If, on the 

 other hand, the Object be opaque, the Lamp should be at a distance about 

 midway behind the eye and the stage; so that its light may fall on the 

 object 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 

 desirable, alike by daylight and by lamplight, to exclude all lateral rays 

 from the eye as completely as possible; and this may be readily accom- 



1 See "Monthly Microsc. Jour.," Vol. xv. (1876), p. 165. As the directions 

 given by these excellent observers for centering the illuminating beam are too 

 long for citation, such as desire to profit by their experience must learn its results 

 from their own account of them 



