"Here is, so far as I know, something new in knife-edge testing:

"In the last stages of testing an optical surface by the knife-edge method of Foucault, it is often found desirable to increase the contrast of the so-called knife-edge shadows without loss of sensitivity due to enlarging the size of the pinhole.

If one should imagine a series of pinholes placed one over the other, together forming what in effect would be a slit, and if the knife-edge were brought up in the usual way until the shadows were most distinct, it would be found that there was no loss in sensitivity, but at the same time that the contrast due to the increased illumination had been enhanced many fold. The proper method of controlling this rather unusual light source will be described here.

"We can state this improved condition in another way: If a slit is used whose jaws are separated so as to let through to the mirror under test no more than the same amount of light that would be delivered by a pinhole, then the slit will be so narrowed that the delicacy of the test is greatly increased.

"The perspective drawing shown will give a clear idea of the whole knife-edge equipment as used here at the California Institute of Technology. The light source is an acetylene flame. Two lenses in the tube adjacent to it form an image of the flame on the adjustable slit placed just in front of a small prism.

"If one now refers to the diagram at B he will see the following situation: A vertical slit (full line) will produce an image parallel to itself-that is, the other vertical full line-by reflection from the concave mirror under test. If the slit is rotated 90 degrees it (and its image) will take the positions given by the dotted lines shown.

"The problem is to tie the knife-edge to the slit in such a manner that the edge of the knife will always be parallel to the image of the slit, no matter what position-angle is used, whether horizontal, vertical or diagonal. Of course, the same purpose can be accomplished by graduating a dial concentric with the center of the slit, and a similar dial for the knife-edge, and setting these at the same angle, thus obviating the gear train. But the relations of slit image and knife- edge may be made automatic by means of the gear train shown in the upper part of the diagram. For anyone who cares to make this automatic control, the characteristics of the different gears are given.

"The slit revolves with the largest gear shown, and the knife-edge with the next largest. The slit and its image are normally about an eighth of an inch on either side of the axis of the mirror, or a quarter of an inch apart.

"The actual test is made just as with the familiar Foucault test and the shadows also are like those of that test. The necessity of rotating the slit and knife-edge arises when testing a flat in combination with a spherical mirror, the cut-offs being horizontal and vertical. Diagonal cut-offs are useful in locating astigmatism. The arrangement for the amateur who is figuring a telescope mirror may be simplified by using as a light-source a small electric lamp covered with ground glass, and the entire instrument may then be rotated as a unit. The eyepiece shown in the sketch is for examining the image of the light-source produced by the mirror as in the eye-piece test."

HERE is another short contribution by Mr. Porter:

"It may be of interest to users of HCF to know that comparison tests have been made here on a number of lenses, half of them polished with a cast iron tool covered with the comb foundation and half with the ordinary tool of pitch facets painted with beeswax. These lenses are commercial plate glass, 19 in number, 7-1/4 inches diameter, plano-convex the convex surfaces all having ,a radius of 12.6 inches.

"Aside from the differences in the tools given above, these lenses received identical treatment on the machine. The photograph shows the machine with two of the lenses on their revolving tables, their respective tools lying face up on the bench. The stroke employed was circular, the tool being driven from a pin set eccentric to the driving spindle, this spindle being offset from the axis of the table below by the desired amount.

"A glance at the work sheets of Mr. Brown, the optician in charge of the work, indicates the following results:

1.-The length of time required to produce a complete polish was about ten hours.

2.-The rapidity of polishing with the two tools was about the same.

"Since the lenses are to be used as condensers only, no figuring of their surfaces has been attempted, the only point I wish to bring out in this run of work being that there seems to be very little difference in the polishing speed between the two tools.

"At first the comb foundation tool had three layers of wax. With this tool the polish came up unevenly from center to edge, depending on slight differences in the radii of the lenses. This, Mr. Brown thinks, is undoubtedly due to the unyielding nature of beeswax. But when the number of layers was increased to six, sufficient air cavities between the different layers permitted good contact to be secured after a period of cold pressing. As matter of incidental interest, the first pairs of lenses were fine ground with No. 303 emery. Later on several pairs were finished with No. 303-1/2, but the time required for polishing was about the same.

"The particular batch of plate glass used contained a quantity of extremely fine bubbles. These bubbles made it somewhat difficult to determine just when complete polish had arrived."

AMONG those enterprising "Amateur Telescope Makers and Astronomers of Tacoma," there is one, Ben L. Nicholson, who has juggled the "makings" of a Cassegrainian and a Gregorian into a neat combination of the two. The idea is sufficiently well shown at the bottom of the opposite page, and a picture of the completed telescope is shown above. This makes Mr. Nicholson a magnate in the "Cassegrainian Club" and we therefore dub him a count-"Count Gregorian Nicholson."

LAST month we told how a snake possessed of a predilection for ladies (it seems they digest more comfortably ) chased a lady twice around Stellafane at the 1931 get-together held there. Several indignant Vermonters now assert that Vermont is as free from snakes as Ireland. But this was no ordinary Vermont reptile, being one of the rare subarctic boa-constrictors. It had stopped off on its seasonal migration from Baffin Land to Tierra del Fuego. During their two yearly crossings of the tropics these snakes find plenty to eat and reach a length of 46 feet. However, by the time they reach Vermont they have shrunk to a mere 16 or 18 feet but they are hungry. The meeting this year will be on August 27. Men bring along snake-bite remedy.

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