28 REPORT— 1861. 



tlie inner and outer portions of his mass, and there must be an approximate convec' 

 tive equilibrium of heat throughout the whole ; that is to say, the temperatures at 

 difierent distances from the centre must be approximately those which any portion 

 of the substance, if carried from the centre to the surface, would acquire by expan- 

 sion without loss or gain of heat. 



Pabt II. On the Origin and Total Amount of the Sun's Heat. 



The Sim being, for reasons referred to above, assumed to be an incandescent 

 liquid now losing heat, the question naturally occurs, how did this heat originate ? 

 It is certain that it cannot have existed in the sim through an infinity of past time, 

 because as long as it has so existed it must have been suifering dissipation ; and the 

 fiiniteness of the sun precludes the supposition of an infinite primitive store of heat 

 in his body. The sim must thei-efore either have been created an active source of 

 heat at some time of not immeasurable antiquity by an overruling decree ; or the 

 heat which he has already radiated away, and that which he still possesses, must 

 have been acquired by some natural process following permanently established 

 laws. Without pronouncing the former supposition to be essentially incredible, 

 the author assumes that it may be safely said to be in the highest degree improba- 

 ble, if, as he believes to be the case, we can show the latter to be not contradictory 

 to known physical laws. 



The author then reviews the meteoric theory of solar heat, and shows that, in the 

 form in which it was advocated by Helmholz*, it is adequate, and it is the only 

 theory consistent with natural laws which is adequate, to accoimt for the present 

 condition of the sim, and for radiation continued at a very slowly decreasing rate 

 during many millions of years past and future. But 7ieither this nor any other natu- 

 ral theory can account for solar radiation continuing at anythi/ig like the present rate 

 for many hundred millions of years. The paper concludes as follows : — " It seems 

 therefore, on the whole, most probable that the sun has not illuminated the earth 

 for 100,000,000 years, and almost certain that he has not done so for 500,000,000 

 years. As for the future, we may say with equal certainty that inhabitants of the 

 earth cannot continue to enjoy the light and heat essential to their life for many 

 million yeai's longer, imless new sources, now imknown to us, are prepared in the 

 gi'eat storehouse of Creation." 



Light, Heat. 

 On Photogj^ajaJik Micrometers. By Sir David Beewstee, K.H., F.R.S. 



When examining, several years ago, some microscopic photogi-aphs executed by 

 Mr. Dancer, the celebrated optician of this city, the author was struck with the 

 singular shai-pness and opacity of some of the lines in such of them as were copied 

 from engravings. The idea occurred to him of obtaining photographically, by means 

 of the camera, micrometrical scales, or systems of delicate lines, opake or transpa- 

 rent, and fitted both for astronomical and microscopical purposes. The suggestion 

 was published in the article "Micrometer" in the ' Encyclopaedia Britannica.' Mr. 

 Dancer had succeeded in making photographic portraits on collodion so small that 

 they were wholly invisible to the naked eye, and 10,000 portraits might be intro- 

 duced into a square inch. The film of collodion upon which these photographs 

 were taken was so thin and transparent that it was invisible, and allowed objects to 

 be seen thi-ough it as distinctly as if it were the thinnest glass. If a system of 

 opake or transparent lines, therefore, was impressed upon collodictti or albumen 

 photogi-aphicaUy, when reduced to the minutest size from a system of large and 

 shai-ply-defined lines, we should have the most perfect micrometncal scale that could 

 be conceived. In the ' Philosophical Magazine ' for August 1861, Dr. Woods, of 

 Parsonstown, had suggested the constniction of photogi-aphic micrometers without 

 being aware of what had been published on the subject. 



* Popular Lecture delivered at Konigsberg on the occasion of the Kant Commemoration, 

 February 1854. 



