THE actual grinding and polishing of small lenses is well described by Porter in the book "Amateur Telescope Making," beginning at page 66. The eyepieces may be mounted without any tests. The objective and each component of the erector should be tested, with crowns facing the light, by the method given at bottom of page 444. The curving of the Ronchi bands, indicating hills or hollows, should be interpreted exactly opposite to the descriptions on page 264, which apply to mirrors. The objective may be figured by making a little pitch tool on a board, and polishing by hand in such a manner as to hit the high spots. The erector lenses are seldom much over a half inch in diameter, and will nearly always be found perfect if made with reasonable care. Should a distinct error be found in one it will probably be best to regrind and polish it. Before testing, the lenses should be "dummy cemented" with glycerine, to take the place of the balsam which will finally be used. Figuring done on cemented surfaces is practically without effect. Try the crown both ways (if it is equi-convex) and leave it the way it will require the least figuring. When the errors are all removed, and the Ronchi bands are practically straight, the lenses are ready to be balsam together.

The balsam sold by most opticians, for cementing microscope cover glasses to slides, is satisfactory. Clean the lenses absolutely spotless, put a drop of balsam on the flint and lower the crown squarely over it. Finally press them together, being careful not to let them slip around on one another, until all surplus balsam is squeezed out of the edges which may be wiped clean frequently with a rag barely moistened with xylol. The lenses are then baked for three or four hours at a temperature just above what the hand can bear.

LITTLE will be said about the mounting, since it would be almost futile to attempt the job without some knowledge and skill in mechanics The mounting presents nothing unusual in the way of lathe work, but the lenses should be rather firmly mounted, with quite a wide rim for a seat, in order to withstand the recoil. The reticule is situated at the focus of the eyepiece, and should be adjustable to and from it, for parallax. Focusing may be accomplished by moving the eyepiece and reticule together, or by moving the erector lenses as a unit, the latter being the prevailing practice. The reticule could be moved by screws or levers for windage and elevation.

Two questions are likely to arise. First, how much distance should be allowed between the erector lenses, if they are separated? Separation of the lenses a certain amount is supposed to improve the color correction under some circumstances, though it has other effects, both good and bad. Practically, it seems to make little or no difference, and the builder may suit his whims, up to half or three quarters of an inch. Secondly, the designer is almost sure to find that the exit pupil diameter and the magnification he picks in the beginning, will require an unreasonably large objective. This is a far more important consideration than the first. Since the second part of the formula given last month for the objective diameter depends only on the width of field, this is another reason for not trying to obtain extremely wide fields. One should make careful drawings, after the plan of Figure 2 (see last month's installment), and study them in order to deduce what changes can he made.

Of course the formula mentioned represents the ideal, with which all parts of the field will be equally bright. Obviously, if all of the rays, when traced backward as in Figure 2 from the stop at F₂, do not succeed in getting through the objective, the only effect will be a slight diminution of brilliance at the edge. With large exit pupils, one could afford to lose perhaps half of the light at the extreme edge of the field, and the effect would not be detectable to a casual user.

It is necessary to cut and try, in designing these systems, and one is almost always forced to make compromises either in the brilliance of the edge of the field or its width, if he is to obtain either large exit pupils or wide fields. It is often difficult for the layman to realize that these systems are always designed either by whim, or by an ordered effort along experimental lines in order to obtain desired ends, and that there are no magical formulas which will answer the question, "What kind of sight do I want?"

THIS concludes Mr. Kirkham's article, but in a subsequent communication to your scribe he recommended that no unusual designs be attempted as the first job. There will be enough grief in the usual types, no doubt, to satisfy most workers.

Commenting at our request for data on spotting telescopes-that is, instruments used by the marksman to inspect his target after firing on the range-Mr. Kirkham writes: "A spotting 'scope is merely a small refractor of rather short focus, equipped with Galilean eyepiece, an erecting eyepiece, or a three-prism erector and standard eyepieces. The Galilean is by far the simplest and cheapest, but not very fancy (small field of view-Ed.). Erecting eyepieces have many bad traits and are not used much now being almost wholly supplanted by three-prism erectors employed in connection with standard eyepieces and this permits the use of ordinary kinds of positive eyepieces, which are highly developed. The worker should make the objective according to the instructions by Ellison in 'Amateur Telescope Making,' and the eyepieces according to Hasting's dope in the same book, and for the erector system, buy cheap prisms, with hypothenuse side twice as long as the diameter of the field lens in the largest eyepiece he wishes to use."

THERE are no short cuts to the consummation of the telescope sight job, and the worker who has omitted to develop background or what might be called "optical gumption'-by doing various jobs of a simpler nature, such as making a reflecting telescope with mirror, will probably sweat more profusely over a telescope sight than the other fellow who has built up some experience over and above that which may be gained from reading books. But the satisfaction of shooting with your own sight would be worth it.

THE following note is from E. Lloyd McCarthy, the same whose focograms appear on page 389 of A. T. M. Mr. McCarthy is now an assistant at the Yerkes Observatory, in Williams Bay, Wisconsin and he writes: "In casting about for simple means of testing a lens have been computing, free from coma and spherical aberration, it occurred to me that the autocollimation test could be performed with vertical set-up and a pan of mercury for the optical flat. Perhaps the idea has been suggested before. I examined the figure of cheap field glass objective by this mean and found that the tool marks were readily visible. Setting the pan of mercury on concrete basement floor, I had no trouble with vibration. If you think the idea is new and any good, it might be welcomed those amateurs who, like me, are forced t economize in their optical work." Mr. McCarthy's set-up is the same as Figure 10, page 121, of A. T. M., with the pan of mercury substituted for the plane mirror and the whole thing turned to a vertical position so that the eye looks downward.

SO many are the separate groups of amateur astronomers and telescope makers around San Francisco Bay that they have organized their various galaxies into o super-galaxy. C. R. Tinsley of Berkeley tells us that the Amateur Telescope Makers San Francisco, under the leadership of Dr. Frances W. Epley of the Flood Building that city, and the Amateur Telescope Makers of Berkeley under the chairmanship Dr. W. P. Bush, American Trust Building Berkeley, California, with clubs in Oakland and other Bay cities, have federated in the Amateur Telescope Makers of the Golden Gate, and will meet quarterly. It will be hard to beat that name.

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