TRANSACTIONS OF THE SECTIONS. 11 



■ On a Real Image Stereoscox^. By J. Clerk Maxavbll, M.A., F.li.S. 



In all stereoscopes there is an optical arrangement, by wliicli the right eye sees 

 an image of one picture and the left eye that of another. These images ought to 

 he apparently in the same place, and at the distance of most distinct vision. In 

 ordinaiy stereoscopes these bnages are virtual, and the observer has to place his 

 two eyes near two apertures, and he sees the united images, as it were, behind the 

 optical apparatus. In the stereoscope made for the author by Messrs. Elliott 

 Brothers the observer stands at a short distance from the apparatus, and looks with 

 both eyes at a large lens, and the image appears a's a real object close to the lens. 

 The stereoscope consists of a board about 2 feet long, on which is placed, first, a 

 vertical frame to hold the pair of pietiu'es, which may be an ordinary stereoscopic 

 sHde, turned upside down ; secondly, a sliding piece near the middle of the board 

 containing two lenses of G inches focal length, placed side by side, with their centres 

 about 1] inch apart; and thirdly, a frame containing a large lens of about 8 

 inches focal length and 3 inches"^ diameter. The observer stands with his e^-es 

 about 2 feet from the large lens. With his right eye he sees the real image of the 

 left-hand picture formed' by the left-hand lens iu'the air, close to the large lens, 

 and -svith the left eye he sees the real image of the other picture fornaed by the 

 other lens in the same place. The united images look like a real object in the air, 

 close to the large lens. This image may be magnified or diminished at plea.?ure by 

 sliding the piece containing the two lenses nearer to, or fmiher from, the pictures. 



Experiments on the Lmmnositij of Pliospliorns. Bij J. Moffat, M.D. 



Heat. 



On some Deductions hj Di: Tyndall from 7m recent Experiments rer/ardinc/ 

 the Radiant and Absorptive Properties of Vapour in the Atmosphere. By 

 R. ErssELL. 



After refen-ing to the importance of Trof. Tyndall's researches on heat as a mode of 

 motion, the author took exception to some of his deductions on the influence which 

 the vapour of water exerts in modifying the intensity of solar and ten'estrial radia- 

 tion. The author stated that he 'had come to the conclusion that the radiant 

 powers of the vapour of water in the atmosphere were not even capable of forming 

 clouds, though they might be capable of forming mists in valleys. In our atmo- 

 sphere he believed that the vapoiu- of water has little power of transmitting its 

 heat into space when it approaches or reaches the dew-point, and that if any cloud 

 had been caused by the radiation of heat into space, its upper sm-face would be flat, 

 like the mists in the meadows before sunrise. These and other reasons led him to 

 the conclusion that the radiation of vapours into space has, directly, a veiy slight 

 influence on the production of rain. 



On a Neiv Telegraphic Thermometer, and on the AppUeation of the Principle 



of its construction to other Meteorological Indicators. By C. Wheatstone, 



F.R.S., n.C.L., LL.D., cjc 



The telegi-aphic thermometer which I constructed in 1843, and which is de- 

 scribed in the Report of the Tldrteenth Meeting of the British Association, depended 

 on the simultaneous action of two isochronous chronometer or cloclc movements — 

 one at the remote statiou regulating the motion of a plunger in the bore of a ther- 

 mometer, and the other at the near or observing station, marking, by the motion 

 of the needle of a galvanometer, the moment at which the contact of the plunger 

 with the mercury of the distant thermometer completed or broke the circuit. The 

 clock movements required to be periodically wound up, and therefore the aflected 

 instnuuent could not be left to itself for an indefinite time. 



There are, however, many situations in which it might be desirable to have 

 moteorologic indications when the instruments would not be accessible for very 



