Visual Efficiency in the Use of Optical Instruments. 275 



Tlie fact that these factors of " sliock " and " strain," which 

 retard clearness and accnracy of vision in proportion to tlie 

 degree of change in the general luminosity, points to a simple 

 manner in which the adjustment of the eye to the new luminosity 

 may be hastened. 



If, on leaving daylight to enter the dark-room with the green 

 safelight, the ordinary artificial light is turned on as well for, say, 

 two minutes, then after turning this out, one can see well enough 

 by the safelight in five to ten minutes. In other M'ords, the 

 twenty to thirty minutes required if we proceed direct from day- 

 light to the dark-room illuminated by the green safelight, has been 

 reduced to seven to twelve minutes by the expedient of taking 

 the contrast in two stages. The strain on the eyes, always naturally 

 greatest at the start, is thereby lessened, and they are, so to say, 

 helped to get on the w^ay. 



On coming into a bright light from a feeble one, the recovery 

 of the eye to perfect adjustment is a much quicker one, for the 

 same amount of contrast, than in the previous one ; but here, too, 

 the same principle holds good, and the recovery may be hastened 

 by subjecting the eyes to an intermediate stage of luminosity. 



These facts lead, indeed, to the somewhat interesting specula- 

 tion whether it might not be possible, by gradually increasing or 

 gradually decreasing luminosity in a number of successive stages, 

 in order to eliminate as far as possible all shock and strain to the 

 eye, to enable it to see by a very much brighter or a very much 

 feebler light than is practicable under ordinary circumstances. 



The " strain " referred to in this paper so far has related to the 

 effort of the eye to see sharply in light of a changed luminosity ; 

 but there is a further potent cause of " strain," in using optical 

 instruments, when the eye endeavours to see sharply that which 

 the instrument has not reproduced sharply, or that which is not 

 well defined in the original. 



This would appear to be a prime cause of inaccuracy in the 

 observations of novices when working with scientific instruments, 

 in many of which the division line between fields, or the scale 

 lines, are wanting in sharpness from too high a magnification, or 

 other avoidable or unavoidable causes. In the Microscope, besides 

 any lack of sharpness in the original or the image, the further 

 factor comes in tliat only one plane of the object is in sharp focus 

 at a time. The novice in his anxiety to see clearly strains the eye 

 in trying to force it to focus sharply beyond its powers of accom- 

 modation, and the fatigue so caused rapidly leads to less instead 

 of greater accuracy. 



This points to the fact that in the use of all optical instru- 

 ments care should be taken to first ascertain and consider whether 

 lack of sharpness of fields, boundary lines, scales or the image or 

 part of the image itself, is avoidable or not, since when such want 



