90 



Cesses, which answer the purposes of teeth. The penis of the male 

 is solely appropriated to the passage of the semen, its external orifice 

 being subdivided into several openings, so as to scatter the semen 

 over an extent of surface, while the urine passes by a separate canal 

 into the rectum. And lastly, the female has no common uterus, the 

 tubes, which correspond to the horns of the uterus in other quadru- 

 peds, receiving the semen immediately from the penis of the male. 



Mr. Home concludes his paper with a conjecture, that more species 

 of this extraordinary animal will be gradually discovered ; a drawing 

 having already been received from Van Diemen's Land of an indi- 

 vidual similar to the Hystrix here described, only with the spines 

 much shorter. And he thinks it probable that the class will in time 

 be found to consist of various kinds, and that those, hitherto known 

 will be arranged under different genera. 



A Method of examining refractive and dispersive Powers, by prismatic 

 Reflection. By William Hyde Wollaston, M.D. F.R.S. Read 

 June 24, 1802. [Phil. Trans. 1802,^. 365.] 



The principle of this method depends on the reflection of light at 

 the inner surface of a denser refracting medium. Its application in 

 the first instance is deduced from a theorem, from which we gather, 

 that since the range of inclination within which total reflection takes 

 place, depends not only on the density of the reflecting prism, but 

 also on the rarity of the medium adjacent to it, the extent of that 

 range will vary according to the difference of the densities of the two 

 media. And that hence when the refractive power of one medium 

 is. known, that of the rarer medium may be thence inferred, by ascer- 

 taining the angle at which the ray of light will be reflected from it. 



Having exemplified this by several instances of different media, 

 the author proceeds to give an account of an instrument he has con- 

 trived for the purpose of applying this principle to practice. Its ob- 

 ject is to measure by mechanical means, which can only be under- 

 stood by inspecting the drawing annexed to the paper, the sine re- 

 presenting the refractive power of the medium applied to the prism. 

 One of the advantages which this method possesses above the usual 

 mode of examining refractive powers is, that whereas hitherto it was 

 necessary that the substances under examination should have two 

 surfaces inclined towards each other under a known angle, this method 

 enables us to deduce the results from only one surface, and without 

 any computation. Hence also, as trials can be made by mere con- 

 tact, may the refractive powers of opake bodies be easily determined. 

 And these powers in different bodies may likewise be used as conve- 

 nient tests in many philosophical inquiries. 



This method applies also to media of which the refractive densities 

 are not uniform, such as the crystalline lens in the eyes of animals, 

 which is now known to be more dense in the centre than at its sur- 

 face. It is here shown in what manner, by placing one of these 

 varied media in contact with a prism, all its gradations of density, 



