Original Document
Original Document
John Brashear on the making of his first 12-inch telescope mirror in 1877

During the winter of 1877 we procured, through Heroy and Marrenner, of New York, glass disks for the reflecting telescope which we had decided to make after my first talk with Professor Langley. We decided to make it of as large diameter as we could handle in the little shop; so we fixed upon twelve inches as the maximum diameter of the glass. It was not difficult to procure the disks, nor were they very costly; so, fearing a repetition of my unfortunate accident in breaking one of the object-glass lenses, we ordered and received two very excellent disks.

A focal length of ten feet was decided upon, and the grinding and fining tool readily prepared for commencing the work. The emery was washed for the different grades, and the disks were cut out of the square plates–just how I cannot remember at this writing. So Ma and I commenced the work that was to take we did not know how long. It all had to be done after my daily work at the mill, and it would not, it could not have been done without the deep and abiding interest of my wife.

Our evenings were frequently interrupted by visitors who wanted to see the heavens in the five-inch telescope which was mounted in what was then the garret of our home, but I do not and did not regret the delay, so great was the pleasure of our visitors at seeing the moon, planets, star clusters, nebulas, and occasionally a comet, in our telescope. But it required many months of labor before we had carried the work far enough along on the reflecting telescope mirror to do the polishing and testing.

Dr. Draper's splendid work, which had been loaned me by Professor Langley, gave full directions for testing the surface or curve of the mirror (Foucault method); and this was supplemented by letters from Dr. Draper himself with whom I had become acquainted by correspondence.

Unfortunately, I had no place where I could make these tests except the open space under the house, and this had to serve the purpose for the completion of the twelve-inch. After many nights of polishing, figuring, and testing, I concluded that the glass was as good as I could make it under the conditions of temperature changes that it had to undergo in working it in the little shop and testing it under the house. One of my greatest mistakes or blunders was that, owing perhaps to the limited time at my command each evening, I did not wait long enough after polishing to see the effects when the increased temperature caused by the polishing had subsided or distributed equally in the mass of the disk. This can be explained to the reader, unacquainted with the delicate methods employed in testing an accurate surface, in this way: if the warm hand were pressed upon such a surface as I am describing for, say, ten seconds, and if the glass was a few degrees cooler than the hand, a cameo of the hand would be raised on the surface. If then the surface were polished before the raised impression of the hand had subsided, this raised figure would be polished off; and then, when the glass would come to a normal temperature, there would be an intaglio or depressed figure of the hand, instead of the raised or cameo figure we had before. This fact is well known to all opticians who have to deal with accurate optical instruments.

But at last our twelve-inch was ready to receive its coat of silver. A splendid tube had been made ready for me by my young friend Edward Klages, a carpenter by trade; and I had made a pattern, and had a casting made, with a temporary though rather light equatorial mounting for it. This was set upon a brick foundation, and a platform built around it, so that observations could be made within our sky limits, which, unfortunately, were restricted in the south and southeast. Nevertheless, we had a fair portion of the sky at command, and were content.

There were several known methods for silvering front surfaces, but at the time my own knowledge of chemistry was limited, and I found that the chemist I consulted could give me but little information on silvering the front surfaces of glass, though quite a number of processes for silvering the inside of mirrors, like reflectors, looking-glasses, etc., were known. Dr. Draper's modification of the Cimeg Process was tried time and again; and as silver was a pretty costly affair for me, I was much discouraged, though at times I succeeded in getting a fair, but not satisfactory, surface of silver.

The mirror had already been tried on the moon with its unsilvered surface, and while its light value was only about one eighth of what it should be, the defining powers were very promising.

My attention had been called by an English friend in the Adams mould shop to a journal printed in London called the "English Mechanic and World of Science"; I borrowed a few numbers and found there a method of silvering by a process requiring heat. So my wife and I prepared to make a trial of the new process. I found everything in readiness when I came home from the mill; and after supper we went to the little shop where the water was soon warmed in the containing vessel by steam heat from the boiler, as rigorous cleanliness in all the processes had to be observed. We had poured the silver solution with its reducer to change it to the metallic form; and you can imagine our delight and joy when we saw a beautiful deposit of silver covering the surface.

But, alas, alas, our joy was soon turned to sorrow, to grief, to keen disappointment that never could be described in words, when we saw and heard our disk crack from edge to center! Not to this day have I determined the real cause of the disaster, although two causes might explain it: unequal heating of the mass of the glass (but this was done so carefully that the second explanation may be more satisfactory) or the possibility that a jet of cold air, coming through a crack in the side of the workshop and impinging upon one side of the glass, cooled it at one point, and hence the rupture.

I do not like to write about this second disappointment in our optical work when we appeared to be just at the climax of success; for this last failure seemed to affect me more than the first one. Failure after all these months, and just when we had reached the goal! What visions I had destroyed in a moment! One of them was the wonderful view I should have of the planet Mars, then coming into the best position for observing that it had been or would be in for years. It was the year 1877, when Professor Asaph Hall discovered the two satellites of the planet.

If I remember correctly, I slept little or none that night, though my dear wife tried her best to cheer me by saying we could finish another glass, as we had both the disk and the experience. I went to the mill the following morning; I walked around like a crazy man; I could not collect my thoughts or concentrate them upon anything. In fact, I think it would have been a Godsend if there had been a breakdown in the machinery that day to take my mind off that broken mirror.

About four o'clock in the afternoon I stopped and pondered for a moment, and this expression came from me, and could almost have been heard, I am sure, had there been any one near me: "What a fool you are, to worry this way; this worry will never mend that broken glass." I am not certain that I was a believer in telepathy then, or that I am now, but somehow I felt in my innermost soul that something was going on at home. I started home as early as possible that evening, and as I climbed the hill it was not with the same heavy heart that I had as I walked down it that morning. As I opened the door I was met with a smile and a kiss, and then dear Ma asked me to go out to the little shop before we sat down to supper. I thought possibly something unfortunate had happened out there. But instead, what did I see? The little shop in prime order, a fire burning under the boiler, engine oiled ready to start, and the extra disk in the lathe ready to have its edge turned with the diamond tool, and its surface roughed out to the approximate curve. Could any one have done more? The memory of that moment, filled with the love and confidence of the one who was more than life to me, I can never forget. To make a long story short, in about two months from that evening, in the early spring of 1878, the new twelve-inch mirror was ready to be silvered.

Source: John A. Brashear, The Autobiography of a Man Who Loved the Stars (New York: Houghton Mifflin Co., 1925).

Back to Top