COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 399 



influence of the lights, after their eyesight had been destroyed. This 

 was accomplished by cutting the optic nerves, a simple operation, 

 performed by inserting the points of a pair of fine scissors just back of 

 the eyeball, through the membrane lining the roof of the mouth, and 

 severing the nerve where it leaves the eye. The frogs did not seem 

 to be greatly inconvenienced by the operation, and three of the four 

 individuals so treated appeared to be as active as before, except that 

 they did not respond so quickly to light. The fourth animal (No. 3 

 in Table X) was more sluggish and behaved much as frogs do that 

 have had the cerebral hemispheres removed. All the specimens were 

 so slow in turning toward the incandescent light which had been used 

 in the previous experiments for orienting the animals into the normal 

 position, that they were placed in the desired orientation by moving 

 the glass plate upon which they were sitting. This could not be 

 rotated directly to bring the frogs into position on account of the 

 compensating circus reflex which resulted, but the plate was moved 

 backward and forward in line with the longitudinal axis of the frog's 

 body a distance of about 15 cm., and each time was turned slightly 

 until the animal was finally oriented. In this way the greater part of 

 the motion was lengthwise of the fi'og, and the rotating was so slight 

 in comparison with the backward and forward movement that there 

 seemed to be no tendency to give the compensating reflex. 



Individuals 1 and 2 usually turned in one direction or the other, or 

 hopped or walked ahead — in other words, gave some locomotor 

 response — within 2 minutes of the time they were exposed to the 

 lights. If not, they were stimulated by touching them from behind. 

 As has been mentioned. No. 3 was inactive and slow to react, while 

 No. 4 was intermediate between No. 3 and the other two. In 

 cases where the animal hopped or walked ahead without turning at 

 least 45° in one direction or the other the record was taken as indiffer- 

 ent, and is included in the column headed in the table. 



The left half of Table X summarizes the reactions to both lights of 

 the four individuals with the optic nerves cut. The larger part of the 

 records (20 out of 36) were indifferent ; but it is a curious fact that 

 each individual turned exactly as many times toward one light as 

 toward the other. In all there were 8 turnings toward each lamp. 

 This shows a perfectly indifferent condition, and demonstrates that 

 Acris is unable to discriminate by means of the skin alone between 

 luminous fields of different size where the intensity of the light fi-om 

 each falling upon the animal is the same. 



In order to make certain that the skins of these animals were sensi- 

 tive to light, the same individuals were finally tested with the small 



