THE MICROSCOPE. 89 



When both eyes are left open and one is applied to an instru- 

 ment, the two images, being unlike, confuse each other in the 

 natural endeavor to blend them. This requires a mental effort to 

 exclude the impression upon the retina of one eye and regard that 

 upon the other only. 



Again, when we close one eye by contraction of the orbicular 

 muscle, or by pressure, as by the hand, we cause contraction of the 

 accommodating muscle also, and of the other eye as well. 



I have proof of this many times each day, while measuring the 

 eye for spectacles, by means of the ophthalmoscope, but we are all 

 familiar with the spasm in both eyes when a particle of dust falls 

 under the lids of one only, and we are conscious of the effort, 

 amounting almost to an impossibility before training, of keeping one 

 eye open and the other shut. 



Both these are at least factors in the fatigue or irritation that 

 accompanies the use of a monocular instrument, and are strong rea- 

 sons for employing a binocular one. 



There are reasons in favor of a monocular microscope, but we 

 need not stop here to discuss the comparative value of the two 

 forms. It is to overcome these two difficulties and to facilitate the 

 training of both eyes that I propose the use of an eye protector. 



A number of these have been devised, among which a plain 

 card, perforated and slipped upon the tube, has, perhaps, been the 

 best; this has to be low down, to be out of the way of the face, and 

 then, to cover the field of vision, becomes so large as to hide the 

 stage, if not interfere with the adjusting screws. 



Another consists of a plate extending horizontally from the ocu- 

 lar. In this the edge has to be cut away to admit the nose, and that 

 necessitates the use of the same ocular and the same eye continu- 

 ously or else demands so much time to remove and replace it that, 

 most operators think it hardly worth the trouble. 



The form that I now propose consists of a small, opaque disk 

 near the eye, supported by a wire extending from its outer edge 

 downward, to a point on the tube low enough to be out of the way 

 of the nose, then bent upward, parallel to the tube, but not touch- 

 ing it, and attached to a ring near the top. 



I made mine of a piece of brass wire. No. 18, about 45 centi- 

 meters long; a loop at one end, 4 centimeters in diameter, covered 

 with a piece of black paper folded over and gummed down, forms 



