Visual Efficiency in the Use of Optical Instruments. 277 



timoimt of light as appertains in the field of view of the instrument 

 would seem to be more effectual. 



My remarks, it should be noted, only deal with the question of 

 prolonged observations, where the main desiderata are continuous 

 accuracy of observation together with the minimum of fatigue or 

 strain, two things which, in fact, as already shown, go together 

 and depend on one another. Where it is merely a question of 

 single or isolated observations, other factors may, and probably do, 

 enter into consideration, so tbat my remarks would not necessarily 

 apply in fall in such cases. 



Instrumciits v:ith Divided Fields : The Spectro-ijhotometer, Photo- 

 meters in general, Polai^imeters. — In these a rhomb is often used to 

 bring the two fields together, and the division lines between the 

 fields is not as a rule very sharp. It is here often better to put the 

 division line distinctly out of focus, and in any case the eye must 

 do its best to ignore the division band. 



These instruments generally suffer somewhat badly, sometimes 

 very badly, from scattered light, which affects accurate observation. 

 Much of this can be eliminated by proper masking, notwithstand- 

 ing the use of brass tubes of small diameter. 



In spectrum work the part not under observation or measure- 

 ment should be cut off, especially if it is the brighter portion. 



Puinge-Jinders. — These being mostly used in the open, and the 

 eyes therefore adjusted to daylight, whilst the field of the instru- 

 ment may be considerably less bright, the case offers some analogy 

 with that of the photographer's dark-room and the external lighting. 



To see with tbe greatest accuracy, constancy, and without strain, 

 it would be necessary to accustom the eyes to the light in the field 

 of view of the instrument before taking observations. As it would 

 be impracticable to keep the eyes glued to the instrument a suf- 

 ficiently long time in advance, it appears reasonable to assume 

 that the use of goggles with neutral tinted glasses which cut down 

 the luminosity of the outside light to approximately that of the 

 field in the instrument would prove serviceable. The ratio between 

 these two luminosities probably bears a more or less constant pro- 

 portion ; when the day was bright the field would be proportionately 

 brighter, and vice versa, so that for any particular range-finder the 

 same pair of goggles would serve under varying light conditions. 



Some simple device would, of course, be necessary, so that 

 when the eyes were placed to the instrument the goggles would be 

 automatically raised, and when they were removed from the instru- 

 ment they would return to their position in front of the eyes, 

 without the admission of extraneous light. By means of ledges on 

 the instrument and on the goggles this should be an easy matter. 



