178 



KNOWLEDGE ♦ 



[Sept. 21, 1883. 



which -will invite our attention, and to which our instru- 

 ment will be directed. Let us see what we may expect to 

 view upon his surface. But here, at the outset, a caution 

 is necessary. On no account whatever must the observer 

 attempt to look at the sun under the same instrumental 

 conditions as he has been viewing the stars. To try to do 

 so without either the interposition of a dark-coloured eye- 

 glass, or the employment of a device to be immediately 

 explained, is almost certain to involve permanent blind- 

 ness altogether. Sir William Herschel lost an eye in such 

 an attempt ; an attempt against which we earnestly warn 

 the student. As a matter of practice, however, opticians 

 send out each astronomical or Iluyghenian eye-piece with 

 a dark-glass cap, which must be screwed on whenever the 

 sun is to be looked at directly through the telescope. 

 Should the purchaser of an instrument have his choice of 

 colour in these eye-caps we would recommend very dark 

 green or blue, or else what is known as " London smoke," 

 as the most agreeable tints for use. Red glasses are less 

 liable to crack with the sun's heat, but they are by no 

 means so pleasant to look through. Whatever colour, 

 however, the observer selects, let him take care that it is 

 dark enough ; and as dark glasses are, as we have hinted, 

 liable to crack with the sun's heat, means must be taken to 

 diminish that heat as much as possible. This will involve, 

 though, one of two things, either the cutting down of the 

 aperture of the instrument to two inches, or even less, if 

 the observation is likely to be a protracted one ; or the 

 turning away the object-glass from the sun at short intervals, 

 should the whole of the object-glass be employed, to 

 give the eye-piece time to cool. There is a device which, 

 should the possessor of a telescope choose to go to the cost 

 of it, enables the sun to be viewed for an almost indefinite 

 period with the whole aperture. In con.sists simply of a 

 perfectly plane plate of glass placed at an angle of 45° 

 with the axis of the telescope, so as to reflect the image 

 formed by the objective in a direction square to the optical 

 axis. The outside of this plate is ground, so as to destroy 

 any secondary reflection ; and, pretty obviously, a very 

 large proportion indeed both of the sun's light and heat, 

 passes through it. The ordinary Huyghenian eye-piece 

 (which may now be covered with a lighter eye-shade) is 

 still employed. Or, finally, we may view the sun without 

 looking through our telescope at all ; and, for getting a 

 general idea of solar detail, the method we are about to 

 describe is perhaps the best of all. Moreover, it enables 

 half-a-dozen people to view the solar disc at once, if 

 necessary. In this way of using the telescope we 

 convert it into a kind of solar microscope or magic- 

 lantern, and throw the sun's image on to a sheet 

 of very tine, clean, hot-pressed cardboard, which we 

 shift to and from the eye-piece, and move the focussing 

 tube until a sharp and distinct image of the sun 

 is obtained. It will be necessary to have a large sheet 

 of pasteboard covered with black paper, through a hole in 

 the middle of which tlie eye-piece comes, in order to 

 shield the card on which the image is projected from 

 direct sunlight. The same end would be more perfectly 

 attained by passing the olject-end of the telescope through 

 an aperture in the shutter of a completely darkened room ; 

 but this is rather too elaborate an arrangement for the ordi- 

 nary observer. Where only one person wishes to see the 

 sun at a time the receiving disc may be fastened at the 

 bottom of a pasteboard cone fitting over the eye-end of 

 the telescope, and with an aperture cut in the side to look 

 through. An arrangement of this sort is illustrated on 

 p. 136 of the " Lessons in Rudimentary Astronomy," by 

 the Editor of Kn'owledge, published by Cassell i Co. 

 (Limited). Whichever of these ways we select to view the 



sun in, we shall be struck by three or four salient features 

 of his surface. The first thing we shall note is that the 

 limb or edge of the sun is perceptibly darker than the 

 middle of his disc, which gradually shades ofl as we 

 approach his circular outline. The eflect of rotundity 

 which this gives to his image is very striking. A little 

 consideration will show that this must be the eflect of 

 an atmosphere surrounding what is technically called 

 the photosphere, or light^radiating surface of the sun. 

 The next thing that will arrest our attention- — perhaps 

 just now the first — wil] be the dark spots which diversify 

 the sun's face. 



Fi-. 1.— Spot on Snn, Sept. 12, 1883, 11.23 a.m. 



The above figure may serve as an illustration of an 

 individual single spot, and was drawn with a power of 

 SO, on Wednesday, September V2, at ll.L'-") a.m. It will 

 be seen to consist of two well-distinguished parts, a dark 

 interior one, known technically as the Umbra (three of 

 these umbrie at least will be observed to be included in the 

 penumbra in the sketch above), surrounded by a lighter 

 fringing which is called the Penumbra. By the use of a 

 peculiarly constructed eye-piece, and a telescope of con- 

 siderable aperture, the late Mr. Dawes discovered black 

 spots within all large umbrre, and even some small ones. 

 If the observer knows exactly what to look for he may 

 sometimes pick these up even with a 3-inch telescope. It 

 will, however, be necessary to cover the diaphragm in the 

 eye-piece with a circular disc of glazed visiting card (with 

 the glazed side towards the field-glass) centrally perforated 

 with a minute hole made with a fine red-hot needle. The 

 telescope is moved until the spot occupies this exceedingly 

 circumscribed field ; and thus cut ofl' from the suiTounding 

 glare, the nucleus may often be detected. We have so 

 far spoken as though spots were isolated, liut they perhaps 

 most frequently appear in groups, involving the most 

 enormous areas on the sun's surface, of the disturbances of 

 which they are the outward and visible sign. 



Our next figure represents a group of spots visible on 

 the sun at 9..50 a.m. on June "Oth of the present year, and 

 was drawn, like every other figure illustrating this series of 



Fig. 2. — Group of Spots, Jane 30, 9.50 a.m. (visible to the 

 naked eye). 



papers, at the telescope. As a reflecting eye-piece was 

 used in this particular case, though, everything is turned 

 right for left in the engraving. It will be noted how the 

 curves of the penumbrw connected the umbra?. Micro- 

 metrical measurement made immediately after our sketch 



