102 Things not generally Know?i. 



For a long time it had been without observer or instruments ; under 

 Napoleon's reign it was demolished. Lacaille never used to illuminate 

 the wires of his instruments. The inner part of his observatory was 

 painted black ; he admitted only the faintest light, to enable him to 

 see his pendulum and his paper : his left eye was devoted to the service 

 of looking to the pendulum, whilst his right eye was kept shut. The 

 latter was only employed to look to the telescope, and during the time 

 of observation never opened but for this purpose. Thus the faintest 

 light made him distinguish the wires, and he very seldom felt the ne- 

 cessity of illuminating them. Part of these blackened walls were visible 

 long after the demolition of the observatory, which took place some- 

 what about 1811. Professor Mohl. 



NICETY REQUIRED IN ASTRONOMICAL CALCULATIONS. 



In the Edinburgh Review, 1850, we find the following illus- 

 trations of the enormous propagation of minute errors : 



The rod used in measuring a base-line is commonly about ten feet 

 long ; and the astronomer may be said truly to apply that very rod to 

 mete the distance of the stars. An error in placing a tine dot which 

 fixes the length of the rod, amounting to one-five-thousandth of an inch 

 (the thickness of a single silken fibre), will amount to an error of 70 

 feet in the earth's diameter, of 316 miles in the sun's distance, and to 

 65,200,000 miles in that of the nearest fixed star. Secondly, as the 

 astronomer in his observatory has nothing further to do with ascertain- 

 ing lengths or distances, except by calculation, his whole skill and arti- 

 fice are exhausted in the measurement of angles ; for by these alone 

 spaces inaccessible can be compared. Happily, a ray of light is straight : 

 were it not so (in celestial spaces at least), there would be an end of 

 our astronomy. Now an angle of a second (3600 to a degree) is a subtle 

 thing. It has an apparent breadth utterly invisible to the unassisted 

 eye, unless accompanied with so intense a splendour (e. g. in the case of 

 a fixed star) as actually to raise by its effect on the nerve of sight a 

 spurious image having a sensible breadth. A silkworm's fibre, such as 

 we have mentioned above, subtends an angle of a second at 3.^ feet 

 distance ; a cricket-ball, 2 inches diameter, must be removed, in order 

 to subtend a second, to 43,000 feet, or about 8 miles, where it would 

 be utterly invisible to the sharpest sight aided even by a telescope of 

 some power. Yet it is on the measure of one single second that the 

 ascertainment of a sensible parallax in any fixed star depends ; and an 

 error of one-thousandth of that amount (a quantity still unmeasurable 

 by the most perfect of our instruments) would place the star too far or 

 too near by 200,000,000,000 miles ; a space which light requires 118 days 

 to travel. 



CAN STARS BE SEEN BY DAYLIGHT ? 



Aristotle maintains that Stars may occasionally be seen in 

 the Daylight, from caverns and cisterns, as through tubes. 

 Pliny alludes to the same circumstance, and mentions that 

 stars have been most distinctly recognised during solar eclipses. 

 Sir John Hersohel has heard it stated by a celebrated optician, 

 that his attention was first drawn to astronomy by the regular 

 appearance, at a certain hour, for several successive days, of a 

 considerable star through the shaft of a chimney. The chim- 

 ney-sweepers who have been questioned upon this subject agree 



