30 



OPTICAL INSTRUMENTS. 



to the Nautical Almanac for 1 787*, says 

 that if 1 of brass, and 1 of silver, be 

 used with only one of arsenic, a most 

 excellent metal will be obtained, which is 

 whiter, harder, and more reflective than 

 any other he ever met with. With re- 

 spect to the practical value of this com- 

 position we cannot speak, but having 

 made specula for reflecting instru- 

 ments ourselves, we can vouch for the 

 goodness of the following, both with 

 respect to the exquisite figure and polish 

 it is capable of assuming, and its free- 

 dom from pores. To make this compo- 

 sition, take two parts of copper, as pure 

 as it is possible to be procured ; (for the 

 goodness of the speculum will depend 

 on the purity of the materials employed) 

 this must be melted in a crucible by 

 itself ; then put in another crucible, 1 

 part of pure grain tint. When they 

 are both melted, mix and stir them with 

 a wooden spatula, keeping a good flux 

 on the melted surface to prevent oxida- 

 tion : this metal must be quickly poured 

 into the moulds, which may be made of 

 founders' loom ; the intended face al- 

 ways being downwards. Where the spe- 

 culum is required particularly good, the 

 best mode of casting is to have an iron 

 mould made with a vertical tube at- 

 tached on one side, and the bottom of the 

 tube to end in a bulb ; the melted metal 

 is then to be poured down the tube, and 

 will fill the bulb and mould, leaving a 

 sufficiency in the tube to give pressure. 

 The bulb being lower than the mould 

 will retain any dense impurities, and 

 the tube the lighter ones, while the spe- 

 culum will be uniform and dense. 



Having thus procured the speculum, 

 the next thing will be to grind it to the 

 required figure ; this is effected on a con- 

 vex brass or hard metal circular tool, 

 carefully turned to a gauge of the required 

 curve. This tool is fixed on a post or ug- 

 right, and the speculum is held in the 

 hand by means of a convenient holder 

 cemented on its back. The grinding is 

 then commenced with coarse emery- 

 powder and water, when the roughness 

 is taken off by moving the speculum 

 across the tool in different directions 

 walking round the post: finer emery 



* This treatise which is now very scarce, is re- 

 publishing in the Technological Repository. 



t It is most probable that the best proportions 

 would be as their respective atomic weight, that is, 

 thrice 32 of copper, and 58 of tin, as the metal would 

 then be more intimately combined. 



is used in the same way, till the surface 

 of the speculum has become uniform. 

 The next step will be to smooth it by 

 means of fine washed flour emery, gra- 

 dually passing from one degree to the 

 next finer, and washing the tool and 

 speculum between each application of 

 emery, to prevent any gritty particles 

 from scratching the metal. When 

 the speculum is completed, and of 

 the required figure, it is next to be 

 polished. This is done either by taking 

 a convex tool similar to the grinder, 

 or the grinder itself, and covering it 

 with pure pitch evenly spread over 

 its surface ; while warm a concave tool 

 of the same figure as the speculum is 

 then worked over its surface wet. 

 When the proper figure is obtained, 

 washed putty (i. <?. combined oxide of tin 

 and lead) is poured on the pitch, and the 

 speculum polished thereon by moving 

 it as before. During the process of 

 grinding and polishing, the tools must 

 be carefully examined by the gauge, and 

 if they happen to get out of the true 

 figure, the speculum must be worked 

 more on the edge, or middle, as the 

 case may require. Instead of the ver- 

 tical post above mentioned, a lap is 

 sometimes employed, which produces a 

 much better figure and more expedi- 

 tiously. A lap consists of a common 

 lathe communicating a slow and regu- 

 lar motion to a vertical mandral, on 

 which the grinding or polishing tool is 

 fixed ; in using the lap, the artist is en- 

 abled to stand in the same place, and 

 has more command over the work. 



Lenses are ground precisely in the 

 same manner as specula, but the po- 

 lishing is different. Here the concave 

 or convex polishing tool is made of 

 brass, and when turned of a proper 

 curve, a smooth thick piece of felt 

 (cloth) is stretched over the tool and 

 cemented to it ; the outer surface is then 

 imbedded with washed putty powder. 

 After this is done, the lens, or block of 

 lenses, is worked on it with cross mo- 

 tions ; if the powder be employed too wet 

 the fibres of the cloth will rise up, and 

 polish not only the surface, but also the 

 small hollow s left in the grinding. This 

 effect, from the nature of the polishing 

 surface being heterogeneous, generally 

 takes place to a greater or less extent 

 when viewed by a microscope ; these 

 cavities being polished admit the light 

 and disperse it, instead of it being col- 

 lected as with a uniform surface. When 





