The majority of amateur observers have, at one time or another, wished for a larger telescope than they possess, but the high price of an instrument of considerable calibre has acted as a deterrent, while such, expressions as figuring, parabolising, zonal testing, have convinced the would-be constructor of a moderate-sized reflector that he personally could never accomplish the task. The writer was in a similar position, and knew very little about mirror-making, until he set out with determination to make a 12|in. mirror, and was rewarded with success.

Difficulty was experienced in obtaining the necessary discs of plate glass, but eventually two of Pilkington's discs, 125/8in. dia., and 11/4 in. thick, were obtained locally for 15/-. After careful inspection, one of the discs free from bubbles, chips or other flaws, was adopted for the mirror, and the other as the tool. A strong barrel, tins of 80, 180, IF., 30 M. Carborundum, a few pounds of common pitch, 1/2lb, jeweller's rouge, and Mr. Allison's book, formed the complement of necessary tools.

The tool was warmed over the gas stove, smeared with turps, melted pitch was poured on the top of the barrel, and the tool pressed on top. The disc selected for the mirror was also warmed, a little pitch poured on the centre and a wooden handle pressed on (Fig. 198). When cold both tool and mirror were as firmly set as Gibraltar.

A little 80 carbo. was strewn on the tool, the face of the mirror dipped in water, and the mirror placed on the tool. Grinding then proceeded by moving the mirror back and fro in straight strokes over the tool, at the same time rotating the mirror on its own axis and walking round the barrel. These motions, especially the walking round the barrel, are necessary to produce a regular curve. At first 1 was too economical with the carbo. and too liberal with the water, resulting in most of it escaping over the edge of the tool ; only a few drops of water should be used, and we get on much better by placing same on the mirror than on the tool. Long strokes were used, at the end of each the centre of mirror nearly reached the edge of tool, while great care was taken to prevent any rocking action which might result in chips. After some three hours, excluding spells for back-straightening, it was considered time to test the curve. The mirror, wiped and flushed with water, was stood on its edge, a lighted lamp was held near the eye and moved back and fro at right angles to the axis of the mirror. If the lamp was nearer than the centre of curvature the reflection would move the same way as the light, if further off the reflection moved the opposite way. The result measured by a tape from mirror to lamp showed the radius of curvature was 10 feet. Grinding was continued until the radius was slightly over 20 feet, when 80 carbo. was consigned to the dustbin, and the tool, barrel and mirror thoroughly swilled and scrubbed with water. Half a dozen or more doses of 180 carbo.. then another scrubbing. IK. Carbo., scrubbing, 30 M. Carbo., followed by a cleaning of the apparatus from all traces of carbo., completed the fine grinding. As each grade was used, the length of the stroke was shortened. The surface of the mirror appeared free from large pits or scratches, and the headlines could be read through it at 6in. distance.

Fig. 198.

The tool was smeared with turps, the mirror coated with rouge and water, and melted pitch, with some added beeswax, poured over the tool. The mirror was laid on top and moved about so that the tool was coated with a layer of pitch the same curve as the mirror. The tool was then divided into 2in. squares, cut with a sharp knife, the centre of tool being in the corner of a square (Fig. 199). The mirror was then moved as in grinding, rouge and water taking the place of carbo. The first sample of rouge was unsatisfactory, but the second produced a high polish after some 12 hours' work. After each half-hour, the tool was allowed to cool, and before testing two hours elapsed.