NATURAL PHILOSOPHY. 143 



glass eye-protector which had been used with similar results. A small 

 portion of the surface of the glass, and to a slight depth below it, of a 

 conical shape, had been so altered by some peculiar action, as to be 

 quite destructive to the correct transmission of the rays of light. A 

 dark red glass in the same place had not been affected, but the heat 

 passed through in such quantity as to be almost painful to the eye. 



THE ZENITH TELESCOPE. 



OBSERVATIONS for latitude in the coast survey, have, for some time 

 past, been made with an instrument called the Zenith Telescope. The 

 mode of observation is that first suggested by Capt. Andrew Talcott, 

 formerly of the U. S. Corps of Engineers, and since practised with suc- 

 cess by many American observers. It consists in observing with a 

 micrometer, the difference of zenith distance of two stars, one north 

 and the other south of the zenith, and making nearly the same angle 

 with it. With the aid of the new Catalogue of Stars, by the British 

 Association for the Advancement of Science, containing the places of 

 over 8000 stars, many such pairs may be selected for any latitude. It 

 is, however, a significant fact in the history of modern progress in 

 astronomy, that it is found necessary to increase the number of stars 

 observed, rather than to multiply observations upon the same pair. 

 Experience has proved, that, upon the average, the probable error of an 

 observation for latitude, with an instrument like that above described, 

 is about one fifth as great as the probable error of the places of the 

 same stars in the British Association's catalogue. The latter are the 

 mean result of observations made at different observatories in Europe, 

 at various periods in the last and present centuries. But the advance 

 in astronomical science, and in means of accurate observation, during 

 the last fifteen years, has been so rapid, that new determinations 

 of the places of the fixed stars are necessary to keep pace with this 

 advance, and are now being made at the National Observatory, in 

 Washington, and other observatories in this country and Europe. 



The discrepancies between the astronomical geodetic differences of 

 latitude and longitude, afford data going to show that differences of den- 

 sity in the strata of the crust of the earth exercise a serious influence 

 upon such observations ; and that the nicer and more elaborate astro- 

 nomical determinations of geographical position may yet become means, 

 in the hands of the geologist, to enable him. to discover the interior 

 character and structure of the earth. 



IMPROVEMENTS IN TELESCOPES. 



AT the Albany meeting of the American Association, Prof. Twining 

 presented a communication on some experimental researches undertaken 

 by him, tending toward improvements in telescopes. He had occasion 

 to make some investigations to determine whether our best glasses did 

 not fall short in the performance of what was expected of them ; and 

 taking the eye as a standard, their recorded penetrative power was 

 found to fall short some two or three orders of magnitude, when pointed 



