SEARCH WORK LIGHT AND THE EYES 417 



blood many times, under favourable conditions, and yet not notice 

 the presence of a white ccrpuscle. and that, too, with one immediately 

 in the centre of the field ; this, moreover, is a large object. 



It is only those in the habit of searching for new things who can 

 appreciate the enormous difficulty in first recognising a new point. 

 Therefore, when critical work is undertaken, ''arc should be exercised 

 to have the conditions as favourable as possible. 



When working with artificial light all naked lights in the room 

 should be avoided. 



It is quite unreasonable to expect the retina to remain highly 

 sensitive if, whenever the eye is removed from the eye-piece, it is 

 exposed to the glare of a naked gas flame. 



At the same time there should be ample light on the microscope 

 table, as it is not at all necessary or desirable that the work should 

 be insufficiently illuminated. All that i> required is that the lamps 

 should have shades and be placed at such a height that the direct 

 ravs do not enter the observer's eye. 



If these precautions are taken, several hours' continued work 

 may be carried on without any injurious effect. 



Some observers use only the left eye. some the right, others the 

 right or left indiscriminately. 



It seems immaterial which is used, it being merely a matter of 

 habit, as those who are accustomed to use one particular eye feel 

 awkward with the other. In continuous work, extending over maii\ 

 months of long daily observation, if the eye has been accustomed to 

 monocular vision, even with high powers, there is no difficulty 

 experienced. The effect "f years of work with optical instruments 

 on those possessed of strong normal sight seems to be an increase 

 in the defining perception accompanied by a decrease of the 

 perception of brightness. Those accustomed to use one particular 

 eye with microscopical work, and who have done much work, would, 

 if they looked at. say, the moon with that eye, see more detail in it 

 than if the other eye were used: at the same time it would not 

 appear as bright. 



If there is too much light, as there often is. when large-angled 

 illuminating cones are used, it is as well to interpose between the 

 lamp and the microscope a piece or pieces of signal green glass ; this 

 softens the light and removes the objectionable yellowness, a feature 

 of illumination not due to the light from the edge of a paraffin lamp, 

 which, as we have stated, is not particularly yellow. Great //<<//// //- 

 ness is a sign of imperfect achromatism in an objective. AVe may 

 with precisely the same conditions find the images yielded by two 

 objectives of the same power and aperture differ, in so much as one 

 is yellow and dim and the other white and bright ; other things 

 being equal, the white and bright image is to be preferred. It is 

 necessary to say ' other things being equal,' because an objective 

 which gives a bright and a white image may nevertheless be inferior 

 to the one giving the yellow and dim picture. Thus if the planes 

 of the lenses of which the objective is composed are not at right 

 angles to the optic axis there will be serious defects in the image, 

 although it is bright and white. This fault is known in practice as 



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