Sun Viewing and Drawing, 433 



two moveable pieces of stout pasteboard (P), or, rather, 

 let us say one piece, cut horizontally into two. At the 

 centre of this line of bisection cut out a semi-circular hole 

 (B) from the upper piece of pasteboard, and a similar corres- 

 ponding one (C) from the lower one ; so that these two 

 holes, when brought together, form a circle, which should be 

 about two inches wider than the diameter of your tube. 

 Round the edges of each of the semicircular orifices sew on 

 (and glue likewise) a thickish piece of opaque vulcanized 

 India-rubber (B and C), so that the elastic material may 

 closely embrace your tube, but still allow your instrument a 

 considerable range, both in altitude and azimuth. It will be 

 far better, too, if the edge of the lower piece of the cardboard 

 is arranged, so that when closed it may overlap the upper 

 piece : otherwise it will be exceedingly difficult to prevent 

 extraneous beams of light from entering through the inter- 

 stices, and seriously interfering with the definition of the solar 

 image on the screen. For this end, indeed, it will be neces- 

 sary that the orifices in both the pieces of India-rubber should 

 be cut rather semi-elliptical than semi-circular. 



It will also be found extremely convenient to have pieces 

 of tape (T T) fastened to the top of the lower piece of card- 

 board, and then passing upwards over, and behind two bottle- 

 corks screwed down to the bars of wood at E, E, and then 

 passing down again, and pinned, when requisite, to the two 

 long, flat pieces of cork glued to the bars at F, F. By this 

 means, after having directed your telescope upon the sun, you 

 may at once effectually close your pieces of pasteboard, with- 

 out any possibility of the lower piece slipping down again, 

 which otherwise would frequently and annoyingly happen. 



Note. — The place occupied by the large vertical hole in 

 the American cloth is indicated in the Fig. by the coarsely- 

 dotted rectangular line, except where its outline is repre- 

 sented, unbroken, between the two temporarily-separated 

 pieces of pasteboard. 



Next prepare your screen. The one employed by the 

 writer is a sheet of " continuous drawing-paper," three and a 

 half feet in length by nearly three feet in breadth, fastened 

 down by a slight wooden frame upon a foundation of mill- 

 board. This is placed on an easel, the legs of which are 

 furnished with holes and pegs, so that the scre6n may be set 

 at any height, as well as also placed at any requisite angle. 

 Provide, also, a large T square, of light wood, by means of 

 which you may at any moment see that the face of the screen 

 is adjusted (and maintained) at right-angles to the line of 

 collimation of the telescope — to the visual axis, that is, of the 

 tube. 



VOL. XI. — NO. VI. 7V 



