SENSE ORGANS AND SENSATIONS 251 



the complete correction of such defects becomes more and 

 more necessary. Those whose occupations require long- 

 continued use of the eyes should see to it that these 

 precious organs are used only under the most favorable 

 conditions and that all strain is as far as possible relieved. 

 13. Accommodation involves nervous as well as muscular 

 work ; the importance of sharp contrast. The work of the 

 muscle of accommodation is controlled by the nervous sys- 

 tem, and accurate accommodation involves an unusually high 

 degree of nervous coordination. The strain thus imposed 

 may be lessened not only by the use of proper lenses and 

 by giving the mechanism of accommodation periods of rest 

 (by looking for a time at distant objects) but also by using 

 the eyes in near work under the most favorable conditions. 

 Perhaps the most important principle involved here is to 

 secure the greatest possible contrast between the light and 

 dark parts of objects at which we are looking. When the 

 contrast is marked, the objects can be seen easily and recog- 

 nized even though the accommodation is not absolutely per- 

 fect. When, on the other hand, the contrast is not great, 

 very accurate accommodation is necessary. Important means 

 of securing the maximum contrast are the following: 



1. The avoidance of too little and of too great illumination 

 of the object. Let the student examine any printed page with 

 different degrees of illumination. The contrast of white and 

 black will be poor in dim and in very bright lights, and 

 greatest with a certain moderate illumination. Hence read- 

 ing in twilight or with sunlight falling directly on the page 

 means greater eyestrain. 



2. The avoidance of a flickering light. A steady light 

 one free from nicker is of the highest importance for near 

 work. In this respect a good kerosene lamp (student's lamp 

 or Rochester burner) is perhaps the best of all lights for 

 reading, provided the heat which it gives off is not too 

 great. Electric lights are good if steady, but too frequently 



