TRANSACTIONS OF THE SECTIONS. 25 



with the journeys turned on them, and all the bearings are of box-wood, which is 

 far better than bell-metal, as neither heating, wearing, or scarcely ever requiring oil. 



In that beautiful machine of Mr. Lassell, the axis of the table which carries the 

 speculum is in the same line with the axis of the slow crank, which by two systems 

 of gearing rotating round a fixed toothed wheel, the pin of the quick crank carries 

 the centre of the polisher with an epicycloidal motion over the surface of the specu- 

 lum. This machine effects the same object simply by a crank rotating in a circle, 

 but the centre of the table which carries the speculum, can be moved at pleasure 

 more or less distant from the centre of that circle. This simple sliding of the axis 

 of the table out of the line of the axis of the crank, causes the centre of the polisher 

 to describe over the face of the speculum the exact figure the more complex machine 

 produces. 



When first I contemplated the construction of a polishing machine on this very 

 simple principle, I never intended to do anything more than to imitate exactly the 

 motions which produced such happy results in the hands of its talented inventor. 

 In carrying out my design, it became obvious, that, by adding three or four more 

 pulleys, at a cost of less than half so many shillings, the machine (in addition to the 

 proved movements of Mr. Lassell's machine) was invested with a power enabling 

 an experimenter in that most interesting branch of practical science, to try the effect 

 of a vast variety of motions for figuring, which the more complex machine is not 

 capable of producing. A few of these motions have been transferred to paper, by 

 substituting for the iron sliding box (which by its pin moves the polisher) a wooden 

 sliding box carrj'ing a pencil, and in place of the speculum these pieces of paper 

 were laid on the table and held there with weights on their corners. The figures 

 are extremely regular and of every conceivable variety of curve. 



Several specula of 4^, 7, and 8^ inches have been repeatedly polished and repo- 

 lished with this machine, and in no instance has a really bad figure been the result. 

 Of course some wpre better than others ; but I believe it will be admitted by all who 

 have trodden this very difficult but interesting path of practical science, that a very 

 fine figure is as much entitled to be enrolled in the chapter of accidents, as a really 

 fine chronometer, which no care in its construction can possibly ensure. If the cause 

 of the imperfection of a speculum be ascertained, and it is found to be decidedly sphe- 

 rical or hyperbolical, the former can with certainty be removed by increasing the 

 excentricity of the table, and the latter by diminishing it. Very frequently, however, 

 it happens that the diflferent zones of a speculum, as tested by diaphragms, have their 

 foci coincident, j'et the speculum does not perform well, from a want of uniformity 

 in the curvature. In these cases I have derived great advantage from placing the 

 centre of the speculum a little excentric as regards the centre of the table, so that in 

 working the excentricity (which is the slow crank in Lassell's) is continually vary- 

 ing from the sum of the two excentricities, to their difference, the mean excentricity 

 remaining unchanged. Latterly, indeed, I have always employed the double excen- 

 tricity. A few remarks upon the formation of the polisher will bring this descrip- 

 tion to a close. Mr. Lassell recommends making the polisher of two pieces of light 

 wood glued together, with their grain at right angles ; in his hands it has certainly 

 performed wonders, but as it is liable to warp with hygrometric changes in the 

 atmosphere, it is as well totally to prevent such warping by employing three, instead 

 of two pieces of board, making the two outside pieces at right angles to the centre 

 piece as regards their grain. To make the furrows in the pitch so that they shall 

 not fill up in poUshing, is extremely difficult by the ordinary process of pressing the 

 pitch while in a soft state with the edge of a ruler, as the pitch forced out of the 

 furrows is heaped up on the edges of the squares, leaving a hollow in the centre of 

 each square ; in working it is forced back again, and it is absolutely necessary that 

 the furrows should remain open during the entire process of polishing. I prefer 

 covering the surface of the polisher with squares of wood about -j- inch in thickness 

 and ^ an inch apart, stuck on with hot pitch or glue, and a nail in the centre. The 

 polisher being held with the face down, the squares are covered over with a brush 

 dipped in the pitch (not very hot), and repeating the operation until a proper thick- 

 ness be obtained ; when made in this way the furrows will never fill up except the 

 pilch be much too soft. 



