Report of the Committee on Colour-Vision. 395 



An instrument based on the principle of Clerk-Maxwell's 

 colour-box could also be used in much the same way as indicated 

 above, but in this case the examiner w T uuld not see the patch of 

 light, and could only examine the case after the positions of the 

 different colours had been accurately determined beforehand. 



Appendix VI. 



Form Test. 



All tests of form- vision depend upon the principle that the 

 magnitude of the image formed upon the retina, by any object, 

 depends partly upon the magnitude of the object itself, and 

 partly upon its distance from the observer ; or, in other words, 

 upon the magnitude of the visual angle which it subtends, while 

 the retinal image must itself attain a certain magnitude before 

 the object from which it is derived can be clearly seen. The 

 precise character of the test object is not important, and perhaps 

 the best is furnished by groups of equal circular dots, each one 

 separated from its neighbours by an interval equal to its own 

 diameter. For all practical purposes, however, printed letters are 

 sufficient, and it is found by experience that capital letters, in 

 block type, are easily distinguished by the majority of mankind 

 when they are placed at such a distance that each limb or part of 

 a letter is seen under a visual angle of one minute, and each 

 letter as a whole under a visual angle of five minutes. Sets of 

 " test-types " were first made on this principle by Dr. Snellen, of 

 Utrecht, and are commonly called by his name. They consist of 

 lines of letters of different sizes, each size marked by a number, 

 which corresponds with the number of feet or metres of distance 

 at which it will subtend the visual angles mentioned above, and 

 at which it should therefore be cleai-ly legible. The acuteness of 

 vision is expressed by a fraction, of which the numerator is the 

 distance of the observer from the tests, while the denominator is 

 the number of the smallest letters which he can read at that 

 distance. Thus if at 20 metres he can read No. 20, he is said to 

 have f£, or normal vision ; but if at 20 feet he can only read 

 No. 40, or if, in order to read No. 20, he finds it necessary to 

 approach within 10 feet, he would, in the former case, be said to 

 have f£, and in the latter of normal vision, in either his 

 vision being equal to f . The test is rapidly applied in practice 

 by hanging up a sheet of propeily constructed letters in good 

 daylight, by placing the person to be tested at a measured 

 distance from them, and by desiring- him to read the smallest he 

 can. The letters may be procured from any optician, and, in 

 testing large numbers of people, it is desirable to have some 

 mechanical contrivance for concealing part of each line, so that 

 the examiner may not be deceived by the lines having been 

 previously learnt by heart by the examinees. 



