

\ XII. — Some Suggestions regarding Visual Efficiency in the Use 

 of the Microscope and other Optical Instruments. 



By J. W. PuRKis, A.R.C.SC. 



{Bead May 17, 1916.) 



The paper which I have the honour to bring before you to-day 



, deals with the relations of the conditions of light prevailing in 



. the field of optical instruments, and that of the light in the 



surroundings where the instruments are used. It attempts to 



show how intimately these factors are connected, the bearing they 



have on the visual efficiency and comfort of the observer, and 



. suggests some simple regulations that seem desirable where accurate 



, observations for a prolonged time are of importance. 



As I have not had any considerable amount of experience with 

 the Microscope, I am keenly alive to the fact that some of my 

 remarks, particularly with regard to that instrument, may not 

 carry general assent. I wish, however, to make it clear at the outset 

 that anything I have to say is only in the nature of suggestions, 

 and if these should act as a stimulus to discussion and lead to 

 interest being aroused on the matter in question, they will have 

 fulfilled their purpose. 



Its importance was first brought home to me as a matter of 

 practical experience. Engaged for a time in working with instru- 

 ments of the spectro-photometer class, measuring densities of photo- 

 graphic plates and absorptions of dyes, and with comparative 

 instruments employing the bi-prism to divide the field for testing 

 the equality of films, I soon found out it was necessary to devise 

 certain precautions, if the observations were to be carried out with 

 accuracy for any length of time, otherwise eyestrain and fatigue, 

 and with these of course diminished accuracy, very rapidly set in. 

 I eventually succeeded in overcoming this, and being able to work 

 for three to five hours on end, without sacrificing accuracy to any 

 very perceptible extent, and without any discomfort or eyestrain, 

 by the expedient of adjusting the light of the surroundings as 

 nearly as might be possible to that prevailing in the field of the 

 instrument. For example, I might be testing dark blue coloured 

 films for two hours, each sheet being tested down each side in 

 turn and then in the centre, thus practically covering the wliole 

 sheet, and during that time the window was shuttered, the door 

 was shut, and an electric lamp was used, well shaded to give only 



