ceases, 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 nterus 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. ‘
The principle of this method depends on the reflection of light at the inner surface of a denser retracting 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, bnt 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 difl'erent 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 surface. It is here shown in what manner, by placing one of these varied media in contact with a prism, all its gradations of density,
