ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 153 



exposure of the collodion the faculae first disappear, then the penumbra round 

 the spots, and lastly the spots themselves. In the photograph the difference 

 in the intensity of the sun's limb and central portions is very marked, but an 

 over-exposure prevents also this from being seen in the photograph. The 

 solar spots and faculae delineated by the Kew Photoheliograph bear exami- 

 nation with a lens of moderate power, and show details not visible to the unas- 

 sisted eye. The faculae and spots are sufficiently marked to make the sun ap- 

 pear globular when two views taken at a sufficient interval are grouped toge- 

 ther in the stereoscope, as will be seen by the slides now before the Meeting. 

 There is not the same difficulty in obtaining stereoscopic pairs of views of the 

 sun as there is in the case of the moon, because any two views taken at an 

 interval of about a day give a perfectly spherical figure in the stereoscope. 

 When the principal spots are near either limb, two views taken at an interval 

 of two days will combine, and even slight changes in the form of the spots 

 do not prevent the perfect coalition of the two pictures. 



Having already most fully described the methods pursued and the pre- 

 cautions to be taken to ensure good results in the case of photoselenography, 

 it will be unnecessary for me here to enter into any details of the chemical 

 part of the processes of photoheliography, for the methods are nearly the same 

 in both cases. So far from seeking a surface less sensitive than ordinary col- 

 lodion, it has been found advisable to use both the bath and collodion in a 

 very sensitive condition, though it is not of course necessary to strain this 

 sensitiveness to the utmost extent for solar photography, as in the case of 

 lunar photography. The bath must, however, be always brought back to its 

 best working state by means of oxide of silver, and subsequent addition of 

 dilute nitric acid in case it has become acid by use. The collodion moreover 

 is used in that condition which photographers would call very sensitive. 



On the Orders of Fossil and Recent Reptilia, and their Distribution 

 in Time. By Professor Owen. 



[A communication ordered to be printed entire among the Reports.] 



With the exception of geology, no collateral science has profited so largely 

 from the study of organic remains as zoology. The catalogues of animal 

 species have received immense accessions from the determination of the na- 

 ture and affinities of those which have become extinct, and much deeper and 

 clearer insight has been gained into the natural arrangement and subdivi- 

 sion of the classes of animals since Palaeontology has expanded our survey of 

 them. Of this the class Reptilia, or cold-blooded air-breathing vertebrates, 

 affords a striking example. 



In the latest edition of the ' Regne Animal' of Cuvier, 1829, as in the 

 * Elemens de Zoologie ' of Milne-Edwards (1834-37), and in the more re- 

 cent monograph on American Testudinata by Prof. Agassiz, 4to, 1857, the 

 quadruple division of the class, proposed by Brongniart in 1802, is adhered 

 to, viz. Chelonia (Tortoises, Turtles) ; Sanria (Crocodiles, Lizards); Ophidia 

 (Serpents) ; Batrachia (Frogs, Newts) ; only the last group is made a di- 

 stinct class by the distinguished Professor of the United States*. In 

 my former Reports on Fossil Reptiles to the British Association, in 1839 

 and 184-1, it was proposed to divide the class into eight orders, viz. — Ena- 



* " After this separation of the Batrachians from the true Reptiles, we have only three 

 orders left in the class Reptiles proper — the Ophidians, the Saurians, and the Chelonians." 

 — /. c. p. 239. 



