166 Intelligence and Miscellaneous Articles. 



Euphrates and the Nile, He mentioned the great tendency to varia- 

 tion in freshwater shells, and the distribution of the same species 

 throughout different and widely separated parts of the world ; and 

 he therefore considered that there was no difficulty in supposing that 

 the Corbicula was contemporaneous in this country with Arctic shells 

 found with it at Kelsey Hill. According to Mr. Jeffreys, specimens 

 of Testacea from the north are larger than those of the same species 

 from southern localities. 



XXII. Intelligence and Miscellaneous Articles. 



PHOTOGRAPHIC MICROMETER. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, Parsonstown, July 1861. 



T SAW last week in some newspaper a notice that in America mi- 

 -*- crometers have lately been made by means of photography. I do 

 not know from what publication this notice has been taken ; but as I 

 have been for the last month or so engaged occasionally in making 

 experiments with the same intention as an original idea, I send 

 you a short account of what I have done, for publication if you think 

 it sufficiently interesting. 



My endeavour was to get a glass slide for a microscope marked so 

 as to measure very minute objects ; and as the micrometer I have 

 (measuring yjjy-th of an inch) was useless ior the purpose I had in 

 view, it occurred to me that by the diminishing power of the camera 

 I might succeed in obtaining smaller divisions. I tried first for pic- 

 tures of dark lines, -j^th of an inch in breadth, on a white ground, 

 reduced to a small compass, but I did not succeed even with a very 

 small aperture to the lens. I then substituted lines |th of an inch in 

 breadth removed to a greater distance, and I got a pretty sharp pic- 

 ture; but I found that the sharpest and best- marked picture of distant 

 lines I obtained was given by opake bars, placed so that the light 

 from a clear sky came to the camera between them. 



By nailing rods of blackened wood, £th of an inch broad and £th of 

 an inch asunder, across a frame, and placing this at a suitable distance 

 with a clear light behind, and using an aperture of about 1th of an 

 inch in diameter, I easily obtained well-marked and sharp lines the 

 t \ ) U th of an inch apart and the 7^0^ °^ an * ncn m breadth, suffi- 

 ciently accurate for all the purposes of a micrometer. The picture 

 of the lines requires to be covered with transparent varnish to pre- 

 vent rubbing. I have taken the picture on very thin talc, and ce- 

 mented it to glass with the collodion between the plates ; and for 

 object-glasses of small power I have found it answer ; but the thick- 

 ness of the talc is too much for the higher powers, as the object 

 viewed and the lines do not sufficiently agree in focus. 



I suppose the reason why lines with spaces between them give a 

 better picture than black lines ruled on a white ground is because 



