214 H. M. JOHNSON 



screen, to about 25% of its original brightness. This made the 

 areas of the two fields approximately inversely proportional to 

 their brightnesses or, in other words, made them very nearly 

 equal in luminous intensity. I gave the dog 100 trials under 

 this condition, but obtained only 61 correct responses. This 

 indicated that he had been merely choosing the field which 

 sent the greater light-flux into his eye, and had not been affected 

 by the difference in area except in so far as it had occasioned 

 a difference in luminous intensity. 



The foregoing results indicate with fair definiteness that the 

 dog did not distinguish the large differences in pattern and in 

 outline presented under the experimental conditions. We are not 

 safe in drawing general conclusions from such evidence, how- 

 ever, as long as there is room for doubt whether a clear image 

 of the test-fields had been formed on the retina. This question 

 hinges on the ability of Dog 1 to accommodate. I have found 

 but few references in the literature bearing on the question of 

 the range of accommodation in the dog. Boden 9 remarks that 

 Wurdinger showed in 1886 that the dog has a ciliary muscle, 

 and that the necessary mechanism for accommodation is there- 

 fore present. He adds that the question of the extent to which 

 accommodation is actually accomplished is one over which 

 opinions differ w T idely. He quotes Hensen and Volckers, 10 who 

 worked on the mechanism of accommodation in young dogs, to 

 the effect that the dog possesses a wide range of accommodatory 

 change — as w T ide at least as the monkey's. On the other hand, 

 he quotes Hess and Heine 11 as having elicited a change of re- 

 fraction of only 1.0 to 1.5 D. by stimulating the sympathetic 

 fibers electrically. Boden himself refracted the eyes of 100 dogs 

 of both sexes and of various ages and breeds, both before and 

 during mydriasis. In the examination without mydriasis he 

 sometimes observed changes in the diameter of the pupillary 

 opening when the dog appeared momentarily to fixate the mirror 

 used in the examination. Boden suspected that an accommo- 

 datory movement accompanied this change, since occasionally 

 the sharp image of the pattern which he projected on to the 



9 Boden, Rudolf. Ueber den Refraktionszustand des Hundeauges. Arch. f. 

 vergleichende Ophthalmologic, vol. I, 1909-10, pp. 195-241. 



10 Cited by Hess. Anomalien d. Refraktion und Akkommodation d. Auges. 

 Grafe-Samisch, Handbuch, 2 Aufiage, p. 230. 



11 Hess and Heine. Grafe-Siimisch, Handbuch, 2 Aufiage, p. 236. 



