THE SENSE OP VISION 629 



was red.' The brightest blue was violet; a less bright, blue; a dirty 

 blue, purple ; and a very impure blue, cerise. 



The trichromic sees red, green, and violet. Cases with a shortening 

 of the red end of the spectrum are unable to see red unless mixed 

 with some degree of orange. Such a case sees shades minus the red 

 rays; therefore, a rose colour, which is a mixture of red and violet, 

 appears violet to him, and more akin to blue than to red. 



Since a system of colour signals is in vogue on sea and land, it is 

 very necessary that persons employed in the marine and railway 

 services should be able to recognize the standard red, green, and 

 white lights under all conditions in which they are likely to be placed. 

 For this reason, it is necessary to have an efficient means of testing 

 colour vision, so that persons whose vision is not satisfactory may 

 be excluded from such services. Generally speaking, such persons 

 are: (1) Those who see three colours or less in the spectrum. (2) Those 

 who have such a degree of shortening of the red end of the spectrum 

 that they are unable to see distant red lights. This is of the greatest 

 importance when it is remembered that in foggy weather the extreme 

 red rays are the most penetrating. (3) Those who, although their 

 vision is normal when close, are unable through insensitiveness of 

 the central part of the retinal apparatus to perceive them at a distance. 



The test most often employed up till now has been the Holmgren 

 wool test. The test colours used are a light green and a light shade 

 of rose, which the candidate is asked to match without naming the 

 colours. A red is also used as a confirmatory test. 



It must be acknowledged that the test is more theoretical than 

 practical. Wool is not a suitable material, and if the dyes used be 

 different, the colours will be different in composition. The colours 

 are also liable to fade, some colours more quickly than others. They 

 also become dirty, particularly the greens, which enables the colour 

 blind to pick them out. Again, the different colours may haye a 

 different feel. The degree of luminosity also varies an excellent 

 aid to the proper identification for passing the test. 



Persons suffering from insensitiveness of the retina will easily 

 pass the test, and others who should not pass for marine and railway 

 work will frequently do so with a little practice with the wools. 



The best test is one where the spectral colours are used, where 

 the luminosity of the light can be varied to approximate the various 

 conditions met with on land and sea, and the subject names the 

 colours which he sees. Such are the lantern and the spectrum 

 apparatus shown in Figs. 356, 357. 



By such means the three groups of cases mentioned above are 

 detected with the greatest ease, while those may be discriminated 

 who would be rejected by the wool test, and yet make useful servants. 

 In testing for colour vision, it must be borne in mind that considerable 

 colour ignorance exists among the .uneducated, especially in regard 

 to shades. 



