82 REPORT — 1844. 



throughout the surfaces will ceasp. Bj- employing a coat of pitch, thicker 

 in proportion as the diameter of the speculum is greater, there will be room 

 for lateral expansion, and the prominence can therefore subside and accurate 

 contact still continue ; however, accuracy of figure is thus to a considerable 

 extent sacrificed. By tlioroughly grooving a surface of pitch, provision may 

 be made for lateral expansion contiguous to the spot where the undue col- 

 lection of polishing powder may have taken place. But in practice such 

 grooves are inconvenient, being constantly liable to fill up ; this evil is entirely 

 obviated by grooving the polisher itself, and the smaller the portions of con- 

 tinuous surface, the thinner may be the stratum of pitch. 



There is another condition which is also important, that the pitchy surface 

 should be so hard as not to yield and abrade the softer portions of the metal 

 faster than the harder ; when the pitchy surface is unduly soft, this defect is 

 carried so far that even the structure of the metal is made apparent. While 

 therefore it is essential that the surface in contact with the speculum should 

 be as hard as possible consistent with its retaining the polishing powder, it is 

 necessary that there should be a yielding where necessary, or contact would 

 not be preserved ; both conditions can be satisfied by forming the surface of 

 two layers of resinous matter of difierent degrees of hardness ; the first may 

 be of common pitch adjusted to the proper consistence, by the addition of 

 spirits of turpentine or rosin, and the other I prefer making of rosin, spirits 

 of turpentine and wheat flour, as hard as possible consistent with its holding 

 the polishing powder. The thickness of each layer need not be more than 

 ^\jth of an inch, provided no portion of continuous surface exceeds half an 

 inch in diameter ; the hard resinous compound, after it has been thoroughly 

 fused, can be reduced to powder, and thus easily applied to the polisher, and 

 incorporated with the s\ibjacent layer by instantaneous exposure to flame. A 

 speculum of three-feet diameter thus polished has resolved several of the 

 nebulae, and in a considerable proportion of the others has shown new stars, 

 or some other new feature; and by the same processes a speculum of six feet 

 diameter has just been completed. 



Report on a Gas Furnace for Experiments on Vitrifaction and other 

 Applicatio7is of High Heat in the Laboratory. 

 By the Rev. William Vernon Harcourt, F.R.S., ^-c. 

 Having commenced in 1834 some experiments on vitrifaction, the object of 

 which was to determine the conditions of transparency in glass, and to com- 

 pare the chemical constitution with the optical properties of difl>rent glasses, 

 I was encouraged by a recommendation which is printed in the 4th volume 

 of the Transactions of the British Association to pursue the subject further. 



I am not, however, prepared at present to report the progress which I have 

 made in these researches, except so far as to give an account to the meeting 

 of the manner in which I have endeavoured to surmount the first great diffi- 

 culty attendant on such inquiries. 



In Dr. Faraday's account of the experiments made in the laboratory of the 

 Royal Institution ibr the improvement of glass for optical purposes (Phil. 

 Trans., 18iiO, part 1.), he has noticed the obstacles which he encountered from 

 the reducing property of the gases produced by carbonaceous fuel, and the 

 contrivance by which he overcame the diflSculty for the particular object 

 which he had in view ; this, however, and other inconveniences from the 

 smoke, the dust, and the cumbrousness of an ordinary furnace, together 

 with the impossibility of regulating the application of the heat and of watch- 



