Visual Efficieiicy in the Use of Optical Instruments. 273 



a small diffused general light to the room by reflection from the 

 wall. This might be followed by an examination of light yellow 

 film, when the shutter would be leant away from the window,, 

 and the door set ajar to get a comparatively bright light from 

 wall and ceilings. In either case the main consideration was to 

 approximate the light outside to the luminosity of the field of the 

 instrument. With the spectro-photometer, taking the saine care 

 to efficiently mask off superfluous light, internal as well as external, 

 I also obtained great benefit both as regards accuracy and comfort. 



This may seem exceedingly simple, and yet the general reasons; 

 and underlying principles, which form the subject of the following, 

 remarks, do not appear to me to have met with sufficient recognition 

 or application in the use of optical instruments generally. 



Broadly speaking, it appears to me that the rapid falling off 

 in accuracy of observations, when these are made under the usual 

 conditions, are mainly the result of the efforts of the eye to adjust 

 and accommodate itself to the more or less rapid succession of 

 "shocks" administered to it by the frequent changes of the general 

 luminosity of the field of vision, and that this kind of eyestrain, 

 or fatigue plays a far greater role than any diminution of the 

 sense of perception by the retina, owing, for example, to the 

 extreme brightness of the objects examined. 



We have to first recognize the principle that the eye adjusts 

 itself comparatively slowly to different luminosities, and, of course, . 

 clearness of sight, and therefore accuracy, depend on the adjust-, 

 ment of the eye being correct. 



Let me illustrate this slowness of adjustment of the eye by . 

 reference to well-known facts. The eye is said to be blinded if; 

 from a dark, that is, a comparatively dark luminosity, by which 

 it has been seeing, it looks at a bright, that is, a comparatively 

 bright luminosity, and then back at the original comparatively 

 dark luminosity. It can now no longer see properly, but takes, 

 a certain time to readjust itself, for the shock to wear off, the time 

 being proportional to the contrast between the luminosities. 



The following table gives a few typical instances where such 

 temporary " blinding" would take place : — 



Dark, or Comparatively Bright, or Comparatively- 



Dark Luminosities. Bright Luminosities. 



1. Bright daylight .. .. .. The sun. 



2 



Interior daylight, as in a church . . Bright daylight. 



3. Interior artificial lighting .. .. An artificial illuminant itself, e.g. 



incandescent wires or mantle. 



4. Photographer's dark-room ... .. Interior artificial lighting. 



5. Clouded moonlight . . . . . . Motor head-lights. 



It will be observed that the same light which is placed under 

 the heading " dark " follows in the next line under the heading 

 'bright," thereby bringing out that "dark" and "bright" are 



