Prof. Tyndall on the Conformation of the Alps. 255 



Apart from the fact that it is entirely unnecessary, it is sure to 

 produce some markings or imperfections. Dust and little hairs 

 adhere to the film while sticky, and when dry it is certain to be 

 either finely corrugated, or else dotted with little transparent 

 elevations, which act like lenses toward the bright beam of light 

 subsequently used. In order to be secure against accidents, I 

 always keep two or three reverses of the finest negatives. These, 

 being copied by contact in rays diverging from a point, are just 

 as valuable as the original. 



With regard to the means used for enlarging, they were at 

 first single and compound achromatic combinations. But these 

 present two defects. The visual and actinic foci are made to 

 coincide, even approximately, with difficulty ; and when in addi- 

 tion a flat field, sometimes 5 feet in diameter, is required, such 

 lenses as could be obtained failed entirely. It then occurred to 

 me to supply their place by a concave mirror of suitable figure, 

 and both difficulties have in consequence been surmounted. 

 The field is flat, as is shown by examination on ground glass, 

 and in the produced photograph. As for the sharpness, I have 

 made a picture of a scale of Lepisma saccharina under a power 

 of 289, by taking a photograph magnified seventeen times, and 

 then magnifying that. It shows the characteristic markings 

 almost as well as a microscope. With this contrivance, the 

 whole interior of the observatory, 27 feet long, can be used as a 

 camera obscura. 



University, New York, 

 September 1, 1864. 



XXX. On the Conformation of the Alps. 

 By John Tyndall, F.R.S., tyc* 



TO the physical geologist the conformation of the Alps, and 

 of mountain-regions generally, constitutes one of the most 

 interesting problems of the present day. To account for this 

 conformation, two hypotheses have been advanced, which may be 

 respectively named the hypothesis of fi-achire and the hypothesis 

 of erosion. Those who adopt the former maintain that the 

 forces by which the Alps were elevated produced fissures in the 

 earth's crust, and that the valleys of the Alps are the tracks of 

 of these fissures. Those who hold the latter hypothesis maintain 

 that the valleys have been cut out by the action of ice and water, 

 thcmountains themselves being the residual forms of this grand 

 sculpture. To the erosive action here indicated must be added 

 that due to the atmosphere (the severance and detachment of 



* Communicated by the Author. 



