C. E. Beecher — Arthrolycosa antiqua. 219 



attracted to a vibrating prong of a tuning fork, is apparently 

 not supported by the new theory that very short electrical 

 wave-lengths are closely related to very short wave-lengths of 

 light : for we should expect that there would be an absorption 

 of these extremely quick oscillations of light or electricity 

 when the attraction between the conductors, insulated by a 

 dielectric, increases. Calling K the specific inductive capacity 

 of the medium between two plane conductors, S the area of 

 the opposed surfaces, c the distance between the plates B 

 and A the potentials of the plates, we have total energy 



Q= - — (B— A) 2 . This evidently increases with the value K. 



The relations between electric conductivity and opacity have 

 been discussed by Maxwell, vol. ii, p. 798. Calling Y the 

 velocity of propagation, K the resistance in electromagnetic 

 measure, of a plate whose length is /, breadth h and thickness 

 z. The proportion of the incident light which will be trans- 

 mitted by this plate will be of the form 



— 47T/J IV 



e ~~b~W~ 

 This proportion evidently increases with the value R. 



Jefferson Physical Laboratory. 



Art. XXX. — Note on the fossil Spider Arthrolycosa antiqua 

 Harger ; by Charles E. Beecher. 



In the March number of this Journal for 1874, Mr. O. 

 Harger described and illustrated a fossil spider from the lower 

 Coal Measures of Illinois,* under the new generic and specific 

 designation of Arthrolycosa antiqua. 



While examining the type specimen in connection with 

 preparing a series of fossils to illustrate Dana's Manual of 

 Geology, it became evident to the writer that all the features 

 of the fossil had not been observed at the time of the original 

 publication. The specimen was then studied and figured in 

 the condition in which it was first discovered, and had never 

 been sufficiently cleaned, nor had the appendages been exposed 

 by removing the superincumbent matrix. This has now been 

 accomplished, and results render it necessary to make some 

 modifications and additions to the original description, as well as 

 to give an illustration of the specimen in its present state. 

 These changes are quite important as Arthrolycosa is one of 



* Notice of a new Fossil Spider from the Coal Measures of Illinois, viii. 219-223. 



