244 Intelligence and Miscellaneous Articles. 



between the two halves of the instrument, which produce a corre- 

 sponding difference in the images, they may be easily detected and 

 allowed for. 



It may be added that a diaphragm may slide at the focus of the 

 eyepiece so as to uncover only equal portions of the two views, and 

 that the adjustment is effected separately for the object-glasses and 

 for each telescope. 



The optical system must exclude both foreign colour and reflected 

 light, conditions which are only realized by means of excellent 

 object-glasses well achromatized and not presenting too pronounced 

 colours of their own. The prisms must also be made of very pure 

 glass, whose colour, as far as possible, should be complementary to 

 that of the object-glasses. Lastly, the magnifying-power should be 

 so small that the telescope may give its maximum light ; for the 

 greater the intensity of the pencil emerging from the eye-glass, the 

 less perceptible will be the inevitable imperfections of the instrument. 

 Hence the diameter of the ring of the eyepiece should be almost the 

 same as that of the pupil. And in order not to introduce a variable 

 element, the minimum (and not the mean) diameter should be taken. 

 In the present apparatus the object-glasses are 54 millims. in dia- 

 meter, and the magnifying-power is 22 times ; so that 2'4 millims. 

 is the diameter of the ring of the eyepiece, while the field of the eye- 

 glass is 1° 26'. 



The means by which the observer may modify one of the two 

 images to make it equal to the other are those employed in the 

 various photometers ; and the present instrument enables any of 

 them to be used at the will of the observer, and according to the 

 object he proposes. The simplest is the use of diaphragms with 

 variable apertures placed before the object-glass ; this is what has 

 been hitherto used, and it has given good results. Each of the ac- 

 cessories may be applied to the two telescopes alternately, or to both 

 at once, as means of comparison and control. 



The instrument may in case of need be used as a general photo- 

 meter ; and as it is provided with circles divided vertically and azi- 

 muthally, as well as a graduated arc to measure the angular distance 

 of the two telescopes, as, moreover, either of these may easily be di- 

 rected to the zenith, it constitutes also an astronomical photometer 

 for measuring the lustre of the stars. Two portions of the sky may 

 also be compared, as to the difference of lustre and colour between 

 them, if care be taken to choose two portions where the atmospheric 

 polarization is almost the same. — Comptes Rendus, June 17, 1867. 



ON THE LAWS WHICH GOVERN THE GENERAL DISTRIBUTION OF 

 HEAT OVER THE EARTH. BY PLINY EARLE CHASE. 



The principal elements of general thermometric variation are (1) 

 the heat imparted by the sun, (2) terrestrial absorption and radia- 

 tion, and (3) atmospheric currents. Of these three agencies the first 

 is, in one sense at least, the chief, since it is the one on which the 

 others depend ; the second is mainly instrumental in modifying the 



