The Seventeenth Century 113 



Several reasons can be presented to account for these shining 

 eyes of fish. One derives from authority, Aristotle's mention of " the 

 head, scales and eyes of fish "; another from the fact that eyes in 

 general appear to glow from the reflection of external light. The 

 belief was general that men whose eyes glowed could see well in 

 the dark. There is also the fact that pressure on the eyeball gives 

 rise to the sensation of light, what is now called a phosphene, de- 

 scribed by Aristotle. If light exists in human eyes, why not in the 

 eyes of animals? Coupled with the fact that luminous bacteria grow- 

 ing on dead fish find the outer surface of the eyeball an excellent 

 medium for growth, there is every reason why Bartholin should 

 stress luminescence of the eyes of fish. 



Bioluminescences were again discussed by Bartholin in the chap- 

 ters on European insects that shine, called by the collective term, 

 " noctilucae." Four kinds were mentioned, two have wings and two 

 others are wingless, crawling on the giound. The relation between 

 the crawling and the flying glowworm, the use of the light and the 

 fact that the egg of the glowworm is also luminous were taken up 

 in detail and have been quoted in Chapter XVI on fireflies and 

 glowworms. 



Bartholin was also aware of the three luminous " insects " men- 

 tioned by Oviedo from the New World, the railroad worm Phrixo- 

 thrix, whose " light shines forth from the joints of their arms, and 

 their head is aglow with no less gleam than that of a burning coal," 

 luminous centipedes, and the cucuyo. 



The third book of De Luce Animalium deals with the cause of 

 the light of living things. It treats of material causes, efficient causes, 

 and final causes in the Aristotelian sense. The material cause is the 

 nature of the matter itself, the efficient cause is what is now under- 

 stood as the cause of physical phenomena, how they arise, and the 

 final cause is the purpose of the light. 



Under material cause Bartholin discussed the differences and 

 qualities of light of living things. Under final cause, it is not sur- 

 prising to find the statement that (Book III, Chap. 9) " Light has 

 been created by God for the perfection of the universe with such 

 beauty and form emulating divinity as he deemed necessary for a 

 perfect example of his glory amidst the mortal and frail. ... In 

 his primary intention he looked toward its usefulness for man. . . ." 



The efficient cause, i. e., the origin of the light, is presented in the 

 form of ten problems: 



I: Is God the immediate author of animal light? 

 II: Does animal light depend on the sky and stars? 



