578 PHYSIOLOGY 



(/) If irregular blind islands called scotomata are found in the visual 

 fields, the cause may be inflammation of the choroid or of the retina itself, 

 or the detachment of the retina from the choroid. 



NIGHT BLINDNESS (group 2) is found in four different types of case. 



(a) As an inherited condition, (b) In diseases of the liver in which bile 

 salts are found to circulate in the blood, since these dissolve the visual 

 purple out of the retina and therefore impair rod vision, (c) As a symptom 

 of insufficient food, (d) In disease of the retina or choroid (e.g. retinitis 

 pigmentosa). In the last two cases it is usual to find colour vision affected 

 to some extent. 



The symptoms of night blindness are well described by the name. The 

 eye does not possess the power of becoming fully dark adapted, and even 

 a moderate degree is only attained after a prolonged stay in the dark. A 

 photochromatic interval is not found, that is to say, when the intensity 

 of a colour is reduced it does not pass through an uncoloured stage (due to 

 the rods). Purkinje's phenomenon is usually poorly developed or not seen 

 at all. There is no cure in the congenital cases. 



COLOUR BLINDNESS. The detection of this condition is important 

 because of the use of coloured signals in the rail way and marine services. The 

 employees of such services should be tested at* stated frequent intervals, 

 because colour blindness may develop (e.g. from tobacco or alcohol poison- 

 ing) in a relatively short space of time. 



The methods of testing colour blindness fall roughly into three 

 classes: (1) those of historic interest; (2) those used by ophthal- 

 mologists for practical tests ; (3) those used for research. Holmgren's 

 wool test is an example of the first class. It consists of a large series of 

 coloured worsteds, the number of different shades being very great. In 

 using the test the doctor hands to the patient selected skeins which have been 

 found by experience to give difficulty to colour-blind persons. He then 

 instructs the patient to select from the box all the skeins that appear to 

 him to have the same colour. His visual defect is judged by the mistakes 

 which he makes. Since coloured wools readily fade and get dirty in use, 

 slips of coloured glass or beads, &c., have been used in a similar way. These, 

 tests are however no longer used, because it is found that colour-blind persons 

 may be able to pass them without detection. 



Of practical tests the lantern test would appear to yield the best results, 

 because'it gives a close imitation of a signal light as seen at different distances 

 and under various atmospheric conditions. In using the test, the doctor 

 shows the patient in turn a series of different colpured lights, and in 

 each case asks him to state what he sees. If he makes no mistakes, the colours 

 are shown through modifying glasses which give the effect of a signal when 

 seen through mist, fog, rain, &c., and the answers noted. One definitely 

 wrong answer should reject the patient, particularly if red, green, or white be 

 one of the signals misnamed. Objections have been raised to this test, because 

 it is possible for a man ignorant of colour names to be failed even if he has 

 normal colour vision. The objection is readily met however. When the 



