Astronomical News. 



103 



Astronomical news. — For some years, a Piedmontese engineer, M. Porro, 

 established at Paris, Boulevard d'Enfer, quite near the Observatory, has 

 been known among physicists and astronomers for his inventive genius 

 and his executive talent. By the instruments which he has made, M. 

 Porro, has caused great progress in two of the most beautiful applied 

 sciences, astronomy and geodesy ; and if his pecuniary resources equalled 

 his fertility of invention he would certainly attain great results. With 

 small resources, he has made these instruments unique in their character 

 which excite at the present moment great sensation. The first is a gigantic 

 telescope, Very simple and not costly, which shows the smallest stars with 

 satisfactory roundness. It has distinctly divided, in a trial by the Frisiani 

 method, two artificial stars of two-tenths of a second diameter separated 

 by an interval of less than one second. 



There has as yet been no opportunity to make sufficient observations 

 with this instrument, but there is great reason to hope that it will bear mag- 

 nifying powers from 1500 to 1800 times, which is more than the immense 

 telescopes of Herschel and Lord Rosse have ever successfully permitted. 



The mounting of so large a telescope would have presented very seri- 

 ous difficulties if the usual system had been followed, and it would have 

 been impossible according to received notions to have made a measuring 

 instrument of it, if M. Porro had not conquered all the difficulties by 

 making the whole telescope, balanced by two counterpoises, revolve around 

 the immovable eye-piece. This construction at once simple and bold, 

 allows the observer to be comfortably placed in a chair, likewise immov- 

 able, whence he can observe all portions of the heavens. 



The natural movements of the instrument as well as the means of 

 measurement are alt-azimuthal, but by a very little simple artifice the op- 

 tical axis of the telescope may at pleasure follow the diurnal movement 

 like an equatorial and give with sufficient precision upon two supplemen- 

 tary coordinates, the stellar coordinates. The astronomer has no occa- 

 sion to leave his chair to read all the circles, the levels, &c. The instru- 

 ment may also at any moment be brought strictly into the plane of the 

 meridian and serve with great precision as a meridian telescope. Not- 

 withstanding the heavy weight and great length of the tube of this in- 

 strument, the azimuthal measurements naturally independent of refrac- 

 tion are here absolute, that is to say, are independent of eccentricity, 

 flexion, &c, thanks to the new and precise arrangements of the maker 

 by means of which the line of vision of the telescope is placed optically 

 in immediate relation with the fixed lines, the meridian and the vertical. 

 The same is true nearly with the apozenith measurements as regards re- 

 fraction. Astronomers know and moreover Sawich has demonstrated that 

 azimuthal measurements alone, independent of refraction, may enter ad- 

 vantageously and to a very great extent into the study of the heavens. 



In a word it is not merely in the extent of its optical power that this 

 instrument is superior to all previously made, but also in the new means 

 of measurement, the precision of which surpasses that of all known instru- 

 ments. The price of the instrument moreover is moderate, within the 

 reach of governments and rich amateurs. It is 160,000 francs. 



The flint glass of this instrument is of the make of Guinand ; the 

 crown glass was furnished by Maes of Clichy. Hitherto the cutting of 

 these glasses has not been done mechanically and yet the degree of pre- 

 cision attained by hand work is not satisfactory. To remedy this M. 



