276 Transaciions of the Society. 



of sharpness cannot be rectified by focusing, or other adjustments, 

 the eye must make the best of a bad job, and should, if possible, 

 avoid the attempt to see such details sharply, thereby causing 

 " strain," and impairing its accuracy for further observations for 

 the time being. 



To see with constancy and maximum accuracy the eye must 

 not be called upon to make any violent adjustments from the 

 normal, and must not be faced with sudden changes of luminosity. 

 If it is, it must be given ample time to adapt itself to the new 

 conditions. This also represents the maximum degree of comfort. 

 The eye should look almost casually at the field, without having 

 to exert itself Looking casually is not synonymous with looking 

 carelessly. What the eye cannot see by quietly looking at an 

 object, it cannot see by what may be called concentrated gaze 

 or stare. 



I come, lastly, to a few observations concerning the use of a 

 few specific optical instruments. They are, as already mentioned, 

 only put forward as suggestions. 



The Microscope. — It would seem desirable in the case of a pro- 

 longed series of observations, when employing a bright field of view 

 in the instrument, to see that the room is also sufficiently brightly 

 illuminated, or, conversely, if tlie room cannot be illuminated to 

 accommodate itself more or less to the field of view in the instru- 

 ment, the illumination of the field itself might be suitably modified. 



It would seem desirable when examining objects with different 

 objectives or eye-pieces in frequent and rapid succession, to avoid 

 great alterations in the luminosity of the field as far as possible, 

 and, if necessary, to attempt to modify the luminosity so as to 

 bring them more closely into correspondence. 



The shading of the illuminant from the eye, in order to avoid 

 superfluous flare, is so generally recognized as to scarcely need 

 drawing attention to, but it may be worth while pointing out that 

 if there is any specially bright portion of the field with which 

 one is not concerned, it would be useful to move it out of the 

 field, if possible, or cover it up, so as to avoid unnecessarily great 

 contrasts of luminosity in the field of observation. Any conscious 

 straining to see parts of an object not in sharp focus should be 

 avoided. 



Lastly, a word on the question of preparing the eye for care- 

 ful observation. It has, I believe, been on various occasions 

 recommended by experienced workers to go in the dark for some 

 time before entering upon careful observations. This somewhat 

 tedious operation would appear to be unnecessary, if it is a 

 question of any continuous observations, as the improvement in 

 visual perception due to resting the retina is counterbalanced by 

 the strain on the eye in accommodating itself to the new con- 

 ditions of luminosity. To rest the eye in approximately the same 



