Remarks on Raihvay and Marine Signals. 127 



focus is formed. If the vertical meridian has a shorter radius 

 of curvature than the horizontal, a point of light in the focus 

 of the latter will not be seen as a point, but as approaching 

 the form of a horizontal line, and vice versa. An eye may- 

 be normal or hypermetropic in one meridian and myopic 

 in another. So, besides deciphering the test types, the eye 

 must also be proved emmetropic — i.e., to possess normal 

 refraction. 



Of course if any of these anomalies of refraction were pre- 

 sent in a high degree, the individual would not be able to 

 read the large test types at the required distance, yet a 

 dangerous amount of ametropia might remain concealed it 

 special attention were not also directed to the state of the 

 refraction. The visual field must also be complete ; there are 

 cases in which, with great contraction of the field, the 

 sharpness of sight remains good in the central parts. 



We now come to the colour perception, which has of late 

 years attracted so much attention. Absolute colour-blind- 

 ness is a very rare condition ; but in the male sex of the white 

 races, diminished colour sense would appear to be of quite un- 

 expected frequency. Thus, according to Mr. George Lawson, 

 Professor Donders, of Utrecht, found amongst 2300 railroad 

 employe's that 152, or 6*60 per cent., were colour-blind. Pro- 

 fessor Holmgren, of Sweden, found amongst 32,165 males 

 that 1019, or 3*25 per cent., were colour-blind. Dr. Cohn 

 found amongst 2429 schoolboys of Breslau 95, or 4 per cent., 

 colour-blind. Dr. Magnus found amongst 3273 school boys 

 of Breslau 3*5 per cent, colour-blind. Dr. Joy Jeffries, of 

 Boston, found amongst 10,387 that 431, or 4149 per cent., 

 were colour-blind. In the female sex colour-blindness seems 

 rare. It is known that for ordinary vision that part of the 

 retina including the macula lutea and its immediate neigh- 

 bourhood is the most sensitive, and that in proportion as 

 images are formed on the more peripheral parts the impres- 

 sion conveyed to the sensorium is less exact and intense. 

 From careful examinations it has been found that blue is 

 distinguished over a larger portion of the visual field than 

 red, and red over a larger part than green. It appears 

 that within the limit of the visual field in the normal 

 eye there is a zone of about 10° in which pigment 

 colours are not recognised. What seems at first sur- 

 prising is that many colour-blind persons (I use the term 

 in the sense before ascribed to it) do recognise and name 

 correctly the principal colours. " Thus," says Professor Pole, 



