NEWTONIAN REFLECTING TELESCOPE. 191 



micrometer, you can ascertain the exact volume of mercury which has 

 been decomposed by a given current in a given lapse of time. That 

 is easily converted into weights, and so you have an exact measure of 

 the effect produced by the current in a certain time. You can perform 

 the whole measurement in five minutes with great facility and great 

 precision, for by a series of experiments it has been proved that the 

 error is not more than "04 per cent, of the quantity measured. If the 

 current is not strong enough crystals may be formed on the surface of 

 the mercury, but that is prevented by heating it a little. The correc- 

 tion for temperature is very small indeed, because the bore being so 

 very small it is scarcely noticeable; but still it can be taken into 

 account, because all the dimensions are known. 



The CHAIRMAN : This ingenious instrument for measuring electrical 

 currents is, I think, likely to prove of great value ; and it is interesting 

 as having been contributed by Mr. Lenz of St. Petersburg, the son of 

 the great physical philosopher of that name. We are very much in- 

 debted to the Baron de Wrangell for his description of it. 



I will now call upon the Rev. Robert Main, the Radcliffe Observer. 



ON A NEWTONIAN REFLECTING TELESCOPE OF SIR W. HERSCHEL. 



The Rev. R. MAIN, M.A., F.R.S. : I am rather taken by surprise in 

 being asked to say anything about this telescope, which I sent from the 

 Radcliffe Observatory, but it may perhaps give me an opportunity of 

 saying a few words to those not accustomed to the instruments in an 

 observatory. The telescope in question is a ten-foot Newtonian re- 

 flecting telescope, and almost the only interest it can have here is that 

 it was prepared and brought down to the Radcliffe Observatory by Sir 

 William Herschel himself, and his correspondence is preserved there. 

 It was made in the year 1812, and was received by the Observatory in 

 1813. I wish I could assure you that there was any series of observa- 

 tions made with it worthy of the telescope. I fancy there were a few 

 casual observations, but nothing much was done with it. 



It will be well perhaps to say a few words about the Observatory itself, 

 and the way in which observatories at that period were furnished with 

 instruments. The want of some such institution in Oxford had- been 



