342 Mr. Westwood's Descriptions of several new British 

 For another example, the experiment of May 23rd gives 

 z = '996759 



and — = 1-49685. 



z w 



Or, the ratio of the velocities appears still to be, when fully 

 considered, nearly 1 *492, as we have taken it to be, at an in- 

 cidence nearly perpendicular. 



These experiments, then, give no adequate ground for main- 

 taining the incorrectness of the hypothesis that the velocity in 

 refracting bodies is inversely as their refractive indices. 



On a careful re-examination of the thickness of the glass, 

 of the value of the threads of the screw by which were mea- 

 sured the distances i i and ef and of the other parts of the 

 apparatus, I could find no ground to suppose the results I had 

 obtained were influenced materially by any other imperfec- 

 tion besides the aberration at the focus of the lens. 



I shall consider it exceedingly important to determine 

 whether, when an achromatic lens of short focus is used to 

 form the luminous point, the central band of direct interference 

 given by two mirrors is black, as it has appeared to me, and 

 to several friends to whom I have shown it, when adequately 

 tried with a common lens, — contrary to what the undulation- 

 ists would have us believe it to be. If the central band shall 

 still be found black, I shall submit to those who are not per- 

 fectly wedded to the undulatory theory, that it must Jail. If, 

 however, when adequately tried, the experiment shall exhibit 

 the central band white, I shall consider one of my greatest dif- 

 ficulties to receiving the undulatory theory to be removed. 



LVI. Descriptions of several new British Forms amongst the 

 Parasitic Hymenopterous Insects. By J. O. West wood, 

 F.LJS.$c.* 



[Continued from vol. ii. p. 445.] 



25. Streblocera, Westw. 



ALYSIM affinis. Caput antice bituberculatum. Antennas fere longitudine 

 corporis 16-articulatae, articulo lmo longo (capite longiori) subtus dente 

 valido armato, 2do brevi apice prioris oblique inserto,3tio paullo majori, 4to 

 etiam oblique inserto, hoc et reliquis filiformibus. Collare angustum. Ab- 

 domen subrhomboideum pedunculo brevi. Alae anticae stigmate magno, 

 areola 1 marginali brevi, areola lma submarginali cum discoidali effusa. — 

 Strebl.fulviceps, Westw. Piceo-nigra, nitida, capite fulvo, oculis ocellisque 

 nigris, antennis fuscis, articulis 3 basalibus fulvis, pedibus fuscis, femoribus 



* Communicated by the Author. 



