Having determined the radius of curvature of surface, both he who grinds the speculum, whether of speculum metal or glass, and he who grinds the object-glass, starts fair; only one has four times the work to do that the other has. The grinding is managed in a simple way, and the process of grinding or polishing an object-glass or speculum, either of glass or of metal, is the same.
Supposing we wish to make a reflecting telescope of six feet focus, or a surface of an object-glass of twelve feet radius, all we have to do is to get a long rod, a little more than twelve feet long, and pin it to a wall at its upper end so that it can swing, pendulum fashion; then at a distance of twelve feet below the point of suspension a pin is stuck through the rod and its point made to scratch a line on a sheet of metal laid against the wall; then this line will be part of a circle struck with a radius of twelve feet. If then the plate be cut along this line we get a convex and a concave surface of the desired radius, and then we can take a block of iron or brass and turn its surface, convex or concave, to fit the sheet of metal or template. For a reflector we should make a convex tool, and for a refractor a concave one.
Generally this grinding tool is divided into squares or furrows all over it, in order that the emery which is used in rough grinding may flow freely about with the water. A disc of glass is then laid on the tool, or the tool on the glass, the two being pressed together by a weight or spring; emery powder, with water, is strewn between them, and one is rubbed over the other by a machine similar to those used for polishing, which we shall explain presently. This operation is continued until the glass is ground all over, and in this process of rough grinding the rough emery is used between the tool and the glass, so that whatever irregularities the glass or tool may have they are got rid of, and it is easy to obtain a spherical surface, and indeed, it is the only surface that can be obtained. Then finer and finer emery is used, till it ceases to be a sufficiently fine substance to use, and a surface of iron or lead is also too hard a surface. Now the polishing begins, and the optician and amateur avail themselves of a suggestion due to Sir Isaac Newton, who always saw much further through things than other people.
Fig. 65.—Showing in an exaggerated form how the edge of the speculum is worn down by polishing.
Even when he first began to make the first reflector, he used pitch, a substance not too hard, nor yet too soft, and one that can be regulated by temperature; therefore for polishing, instead of having a tool made of metal, pitch laid on glass or wood and supplied with rouge and water is used. This polisher of pitch is divided into squares by channels to allow free flow of rouge and water, and is laid on the mirror or object-glass, or vice versâ, and moved about over it.
When the maximum of polish is attained the work is done, and the object-glass finished, as here we have to do with a spherical surface. In the grinding of the two discs for Mr. Newall’s telescope 1,560 hours were consumed, the thickness of the crown disc having been reduced one inch in the process.
In the case of specula, however, there is more to be done; and it is in this polishing of specula that the curve is altered from a circle to a parabola by using a certain length of stroke, size of polisher, consistency of pitch, and numbers of other smaller matters, the proper proportionment of which constitutes the practical skill of the optician, and it is in accomplishing this that the finest niceties of manipulation come into play, and the utmost patience is required. 1,170 hours were occupied in the grinding and polishing of the four-feet Melbourne speculum. This was equivalent to 2,050,000 strokes of the machine at 33 per minute for rough and 24 for fine grinding. Dr. Robinson, in his description of the grinding operations, states that at the edge of one of the four-feet specula the distance of its parabola from the circle was only 0·000106˝.
In the early times of specula the polishing was invariably done by hand, a handle being cemented by pitch to the back of the speculum to work it with. Mudge tells us that at first, when the mirror was rough from the emery grinding, it was worked round and round on the pitch, which was supplied with rouge and water and cut by channels into small squares, carrying the edge but little over the polisher, an occasional cross stroke being made. The effect of this was to press the pitch towards the centre where the polish always commenced, and gradually spread to the circumference. As soon as the polishing was complete the speculum was worked by short straight strokes across the centre, tending to bring it back to a sphere; then the circular strokes were recommenced to restore the paraboloid form: these were continued for a short time only, otherwise it would pass the proper curve and require reworking with straight strokes again. By this method some small mirrors of first-class definition were constructed.