THE small observatory shown at the right was made by Albert F. Schroeder, 1175 Forest Cliff Drive, Lakewood, Cleveland, Ohio, and will be described in detail in a later number.

THE telescope shown below right is a four incher, made by Arthur D Flagg, from easily obtained materials, the mirror disk being a piece of broken windshield from a motor car. Mr. Flagg used part of an old Ford water pump for his polar axis support. He also used the brass shells of discarded light sockets for making three eyepieces. A magnification of 40 or more diameters may be had from such a telescope.

P. M. JONES, 3421 Second Street, Des Moines, Iowa, whose second telescope is illustrated opposite, used the H C F lap and "found it all that was claimed for it, both in its rapid cutting and the east of changing the tool to alter the figure." The framework of his mounting is of steel angles. This is very rigid. The yoke is made of three-inch channels welded at the joints. The telescope has a friction drive on both axes, consisting of a 10-inh disk edged to fit a small V-shaped roller held against it by a spring. Setting circles were made of lacquered protractors costing 20 cents each.

WE now continue and conclude below Professor Douglass' informative discussion of "Atmosphere, Telescope and Observer," begun in the last number:

"It is easily a matter of observation that making allowance for the variation in brilliancy of the apparent field when the eye is in the focus, the atmospheric currents are precisely the same in telescopes of different apertures at the same time and place. This is of course what should be expected. But different apertures do change the character of the seeing and this also is what we expect. Conceiving the waves to consist of crests and valleys as the waves on water, we see that the refraction takes place on the slopes between these and that two adjacent slopes refract in opposite directions. If we take the distance from crest to crest as d and the mean amount of refraction in each slope as r seconds of arc we shall find that in a telescope with an aperture of 1/2 d or less the image in the focus will oscillate through a distance of 2 r. If the aperture of the telescope is d we would see in succession, if the waves were all of perfect form, first a haziness of the planet, then a displacement of r seconds in one direction, then a haziness followed by a displacement of r seconds on the other side of its original position, then a haziness as at first, and so on; the haziness in each case being due to the presence of two slopes at once before the lens. If the aperture were 1-1/2 d there would be alternations of haziness with these displacements of r seconds, the displacements themselves being not entirely free from haziness. With further increase of the size of the objective, displacements would for a time exist but become more and more hazy until at last they would cease, leaving the planet perfectly steady but blurred.

"Such is the effect of using different apertures. As a matter of fact we rarely have such simple conditions in actual experience. We have a given telescope and usually three series of air waves which may all be of different sizes. By a big diaphragm we can get rid of the blurring effect of the largest set. By medium and small diaphragms we can improve successively the bad effect of the other series but in doing so the light is enormously decreased. We may summarize this matter of aperture by saying that the smaller the aperture the more bodily motion and less confusion of detail; the larger the aperture, the less bodily motion and the more confusion of detail.

"Good seeing then, apart from transparency of the air, consists of two factors, steadiness and definition. In a given atmosphere these factors vary with the aperture, one being improved at the expense of the other; either one may come from a superior atmosphere.

"Aperture has another effect on the seeing which is of different kind, namely, physiological. It principally concerns observers of planetary detail. All the effects of this kind observed, vary with the size and brilliancy of the pencil of light entering the eye. The first imperfections noticed are motes which float about and persist in coming upon the planet which is under examination. They can also be seen against a clear blue sky.

"Perhaps the most harmful imperfection in the eye is the lack of homogeneity within the more dense transmitting media, either the lens or the membranes; probably the former. Under proper conditions the lens (presumably) displays irregular circles and radial lines, the whole resembling a spiderweb structure. Under actual tests this structure is so very prominent that we wonder why the eye is able to give such good definition as it does. No optician could ever sell a lens so badly made, except for the coarsest usage; in proportion to its size it has the imperfection one finds in the lens of a bull's eye lantern."

Professor Douglass next describes methods of examining one's own eyes, but this is too long to quote here.

"High powers greatly reduce contrast, he continues "When one changes from a low to a high power the light parts of the planet become correspondingly fainter but the dark parts seem to become lighter.

"The eye has considerable power of adapting itself to contrast occurring in different intensities of light in a manner entirely independent of the size of the pupil. This has often been exemplified in the experience of visitors looking at Mars, when the emergent pencil was much smaller than the pupil of the eye; at first they see nothing but a glare of light but after looking sometimes for 12 to l0 minutes the glare diminishes and markings begin to appear. This is a certain power of adaptation which I have never seen mentioned before. After much practice that first glare becomes less and less noticeable and the eye becomes more sensitive to the particular range of contrast sought. That, in fact, is the training required by the eye to discern planetary detail, and for different planetary bodies which present different degrees of contrast and different intensities of light the training has to a certain extent to be undergone afresh in each case "

FROM Professor Douglass' article the reader will have gathered some of the basic principles of good seeing and bad. It is quite natural that the beginner should assume that magnifying power, or rather actual seeing power, should be increased simply by increasing the aperture of the instrument used. The matter is not, however, so simple as that; it involves a number of variable factors, variable not only from night to night but from place to place. At best any telescope is a compromise-a compromise between a number of these factors. The most elementary point for the very beginner to grasp is that mere magnifying power does not necessarily guarantee good seeing. On many occasions one can crowd on higher magnification by means of higher powered eyepieces and actually see less than with a low power. Fully to understand the considerations Professor Douglass discusses naturally requires years of patient observation under wide variety of circumstances.-A.G.I., Tel. .Ed.

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