114 A CHAPTER ON LIGHT AND COLOUR. 



containing granules, and finally we find the Bacillary T.ayer, 

 which consists of a set of elongated bodies, arranged radially, and 

 closely set together. These are of two kinds, known as Rods and 

 Cones. It is believed that the distance between two neighbouring 

 rods or cones determines the smallest angle between two bodies 

 which are just seen as two. The perception of light is due in 

 some way to the presence of the bacillary layer, for it is tolerably 

 certain that the nerve fibres overspreading the retina are incapable 

 of being directly excited by light vibrations. The general surface 

 of the outer ends of the rods and cones of this layer is in contact 

 with a layer of cells containing a black pigment, which is supposed 

 to serve the purpose of absorbing stray rays of light. 



Californian Trap-door Spider.— At a meeting of the Academy 

 of Natural Sciences of Philadelphia, held June 23rd, 1896, the 

 Rev. H. C. Cooke reported a series of observations on the Califor- 

 nian Trap-door Spider, Cteniza Californica, made by Dr. Davidson, 

 who had been able to determine the time required for the construc- 

 tion of the burrow in confinement, and other matters connected 

 with the life-history of the animal. It had taken ten hours to 

 construct the nest with the hinged door, another spider having 

 made a hole large enough to conceal itself in two hours. The 

 method of digging was the same, in the main, as that described 

 by the speaker for the tarantula. The young, when they emerge, 

 at once build their own miniature nests, which are renewed every 

 spring, until they reach the full size. Based on his study of a 

 Lycosid, the speaker had predicted that the enemy of the Trap- 

 door Spider would be found to be a diurnal wasp. Dr. Davidson 

 has established the fact that such is the case, and that the attack- 

 ing species is Parapoviphilio planatus, Fox. — Science. 



