This box should be made of black cloth or calico fastened over a light framework of wire or cane. The base of the pyramid should be covered on the inside with a sheet of white glazed paper, or with some other uniform white surface. Captain Noble, I believe, makes use of a surface of plaster of Paris, smoothed while wet with plate-glass. The door b c enables the observer to "change power" without removing the box, while larger doors, d e and g f, enable him to examine the image; a dark cloth, such as photographers use, being employed, if necessary, to keep out extraneous light. The image may also be examined from without, if the bottom of the pyramid be formed of a sheet of cut-glass or oiled tissue-paper.
When making use of the method just described, it is very necessary that the telescope-tube should be well balanced. A method by which this may be conveniently accomplished has been already described in Chapter [I.]
But, undoubtedly, for the possessor of a moderately good telescope there is no way of viewing the sun's features comparable to that now to be described, which has been systematically and successfully applied for a long series of years by the Rev. F. Howlett. To use his own words: "Any one possessing a good achromatic of not more than three inches' aperture, who has a little dexterity with his pencil, and a little time at his disposal (all the better if it be at a somewhat early hour of the morning)" may by this method "deliberately and satisfactorily view, measure, and (if skill suffice) delineate most of those interesting and grand solar phenomena of which he may have read, or which he may have seen depicted, in various works on physical astronomy."[15]
The method in question depends on the same property which is involved in the use of the pyramidal box just described, supplemented (where exact and systematic observation is required) by the fact that objects lying on or between the lenses of the eye-piece are to be seen faithfully projected on the white surface on which the sun's image is received. In place, however, of a box carried upon the telescope-tube, a darkened room (or true camera obscura) contains the receiving sheet.
A chamber is to be selected, having a window looking towards the south—a little easterly, if possible, so as to admit of morning observation. All windows are to be completely darkened save one, through which the telescope is directed towards the sun. An arrangement is to be adopted for preventing all light from entering by this window except such light as passes down the tube of the telescope. This can readily be managed with a little ingenuity. Mr. Howlett describes an excellent method. The following, perhaps, will sufficiently serve the purposes of the general observer:—A plain frame (portable) is to be constructed to fit into the window: to the four sides of this frame triangular pieces of cloth (impervious to light) are to be attached, their shape being such that when their adjacent edges are sewn together and the flaps stretched out, they form a rectangular pyramid of which the frame is the base. Through the vertex of this pyramid (near which, of course, the cloth flaps are not sewn together) the telescope tube is to be passed, and an elastic cord so placed round the ends of the flaps as to prevent any light from penetrating between them and the telescope. It will now be possible, without disturbing the screen (fixed in the window), to move the telescope so as to follow the sun during the time of observation. And the same arrangement will serve for all seasons, if so managed that the elastic cord is not far from the middle of the telescope-tube; for in this case the range of motion is small compared to the range of the tube's extremity.
A large screen of good drawing-paper should next be prepared. This should be stretched on a light frame of wood, and placed on an easel, the legs of which should be furnished with holes and pegs that the screen may be set at any required height, and be brought square to the tube's axis. A large T-square of light wood will be useful to enable the observer to judge whether the screen is properly situated in the last respect.
We wish now to direct the tube towards the sun, and this "without dazzling the eyes as by the ordinary method." This may be done in two ways. We may either, before commencing work—that is, before fastening our elastic cord so as to exclude all light—direct the tube so that its shadow shall be a perfect circle (when of course it is truly directed), then fasten the cord and afterwards we can easily keep the sun in the field by slightly shifting the tube as occasion requires. Or (if the elastic cord has already been fastened) we may remove the eye-tube and shift the telescope-tube about—the direction in which the sun lies being roughly known—until we see the spot of light received down the telescope's axis grow brighter and brighter and finally become a spot of sun-light. If a card be held near the focus of the telescope there will be seen in fact an image of the sun. The telescope being now properly directed, the eye-tube may be slipped in again, and the sun may be kept in the field as before.
There will now be seen upon the screen a picture of the sun very brilliant and pleasing, but perhaps a little out of focus. The focusing should therefore next be attended to, the increase of clearness in the image being the test of approach to the true focus. And again, it will be well to try the effect of slight changes of distance between the screen and the telescope's eye-piece. Mr. Howlett considers one yard as a convenient distance for producing an excellent effect with almost any eye-piece that the state of the atmosphere will admit of. Of course, the image becomes more sharply defined if we bring the screen nearer to the telescope, while all the details are enlarged when we move the screen away. The enlargement has no limits save those depending on the amount of light in the image. But, of course, the observer must not expect enlargement to bring with it a view of new details, after a certain magnitude of image has been attained. Still there is something instructive, I think, in occasionally getting a very magnified view of some remarkable spot. I have often looked with enhanced feelings of awe and wonder on the gigantic image of a solar spot thrown by means of the diagonal eye-piece upon the ceiling of the observing-room. Blurred and indistinct through over-magnifying, yet with a new meaning to me, there the vast abysm lies pictured; vague imaginings of the vast and incomprehensible agencies at work in the great centre of our system crowd unbidden into my mind; and I seem to feel—not merely think about—the stupendous grandeur of that life-emitting orb.
To return, however, to observation:—By slightly shifting the tube, different parts of the solar disc can be brought successively upon the screen and scrutinized as readily as if they were drawn upon a chart. "With a power of—say about 60 or 80 linear—the most minute solar spot, properly so called, that is capable of formation" (Mr. Howlett believes "they are never less than three seconds in length or breadth) will be more readily detected than by any other method," see Plate [7]; "as also will any faculæ, mottling, or in short, any other phenomena that may then be existing on the disc." "Drifting clouds frequently sweep by, to vary the scene, and occasionally an aërial hail- or snow-storm." Mr. Howlett has more than once seen a distant flight of rooks pass slowly across the disc with wonderful distinctness, when the sun has been at a low altitude, and likewise, much more frequently, the rapid dash of starlings, which, very much closer at hand, frequent his church-tower."
An eclipse of the sun, or a transit of an inferior planet, is also much better seen in this way than by any other method of observing the solar disc. In Plate [7] are presented several solar spots as they have appeared to Mr. Howlett, with an instrument of moderate power. The grotesque forms of some of these are remarkable; and the variations the spots undergo from day to day are particularly interesting to the thoughtful observer.