594.] The Refractive Character of the Eyes of Horses. 417 



Of myopia, Berlin found in one case as much as 3 D, and in another, 

 75 D. 



Hirschberg, adopting the same method of enquiry as Berlin, viz., 

 lirect examination of the fundus with the ophthalmoscope, found a 

 jnsiderable amount of astigmatism in the eyes of horses. 



Lang and Barrett, in the six eyes they examined, found one was 



imetropic, three hypermetropic and astigmatic, and two had slight 

 lixed astigmatism. The astigmatism only once equalled 1 D, and 

 le horizontal was always the least curved meridian. 



In the enquiry I am recording, the number of horses examined 

 54 ; these 54 horses, from one cause or another, furnished me 

 nth 100 eyes. 



Of the 54 animals, 31 were mares and 23 were geldings ; their ages 

 varied from 4 years to 16 years ; the largest number of horses of one 

 age (5 years) was 13, 7 horses were 6 years old ; 7 horses 7 years , 

 6 horses 8 years ; the remaining numbers for each age varied from 

 1 to 4. 



Before recording the results of this enquiry, I think it necessary to 

 describe the method by which retinoscopy is carried out on the 

 horse. 



It has been shown by Messrs. Morton and Barrett* that the results 

 of retinoscopy are liable to considerable error unless practised at the 

 right portion of the fundus. 



The observations made by these and other observersf are of the 

 greatest value, for they have drawn attention to the difference in the 

 reflex obtained in the human subject at the yellow spot and at the 

 optic disc, and have settled that retinoscopy is only reliable when 

 jractised at the visual axis of the eye. 



In the horse I found very early in the enquiry, that confusing and 

 inflicting results might be obtained with the same eye when reti- 

 noscopy was practised at different parts of the fandus, for the reflex 

 given at the optical axis was not the same as that obtained when the 

 fundus was looked at obliquely. "We have no knowledge whether 

 the optical axis and visual axis of the horse are identical for the 

 purpose of this enquiry it has been assumed that they are. 



The horse has no yellow spot, so that a valuable guide is lost in 

 determining the position on the fundus at which to judge the reflex ; 

 on the other hand, the error of taking the reflex at the disc is not 

 likely to occur, owing to the difficulty of seeing the disc with the 

 head in the ordinary position. 



* "A Clinical Investigation of the Methods of Practising Retinoscopy," ' British 

 Medical Journal,' ICth January, 1886. See also 'Refraction of the Eye,' A. S. 

 Morton, M.B., F.R.C.S. 



t ' The Shadow Test in the Diagnosis and Estimation of Ametropia.' W. B. 

 Beaumont. 



