Royal Society. 305 



which is able to get through both media, it necessarily remains the 

 same as before. In the case of objects which are only feebly fluo- 

 rescent, it is sometimes better to leave the second medium in its 

 place, and use a third medium, called by the author the transfer 

 medium, which is placed alternately in the path of the rays incident 

 on the object-end of the rays coming from it to the eyes. 



Independently of illumination, the change of colour corresponding 

 to the change of refrangibility, and the difference of colour with 

 which the object appears, according as the transfer medium, or the 

 complementary absorbent used as a transfer medium, is held in front 

 of the eyes or in front of the hole, afford in most cases a ready mode 

 of detecting fluorescence. 



Instead of trusting to the absolute appearance of the object, it is 

 commonly better to compare it with some fixed standard. The 

 standard substance ought to be such as to scatter freely visible rays 

 of all refrangibilities, but not to give out rays of one refrangibility 

 when influenced by rays of another. The author employed a white 

 porcelain tablet as such a standard ; and the object to be observed 

 was placed on the tablet, instead of being laid directly on the black- 

 ened shelf. 



Another mode of observation consists in using a prism in combi- 

 nation with the principal absorbent. The object being placed on 

 the tablet, a slit is held close to it, in such a position as to be seen, 

 projected partly on the object and partly on the tablet, and the slit 

 is then viewed through a prism. The fluorescence of the object is 

 evidenced by light appearing in regions of the spectrum, in which, 

 in the case of the rays coming through the principal absorbent, and, 

 therefore, in the case of the rays scattered by the tablet, there is 

 nothing but darkness. 



The author states that these methods proved to be of such deli- 

 cacy, that, even on an unusually gloomy day, he was able readily to 

 detect the fluorescence of white paper ; and even in the case of sub- 

 stances standing much lower in the scale, the fluorescence could be 

 detected in a similar manner. 



In conclusion, the author states that he had found the property 

 of fluorescence to belong to a peculiar class of salts, the platinocya- 

 nides, making a third instance in which this property had been 

 connected with substances chemically isolated in a perfectly satis- 

 factory way. The present instance opens a new field of inquiry in 

 relation to the polarization of the fluorescent light. 



"Researches in Embryology; a Note supplementary to Papers 

 published in the Philosophical Transactions for 1838, 1839 and 1840, 

 showing the confirmation of the principal facts there recorded, and 

 pointing out a correspondence between certain structures connected 

 with the Mammiferous Ovum and other Ova." By Martin Barry, 

 M.D., F.R.S., F.R.S.E. 



Referring to his account of the process of fecundation of the mam- 

 malian ovum and the immediately succeeding phenomena, published 

 in various papers in the Philosophical Transactions, the author calls 

 attention to the confirmation which his views have received from 

 corresponding observations made by subsequent inquirers on the ova 



Phil. Mag. S. 4. Vol. 6. No. 39. Oct 1853. X 



