68 On the Solar Spots. 



to 11 sec. 12 deg. and will twice a year decline from it the 

 greatest possible quautity, being when his long, is 2 sec. 10 deg. 

 and S sec. 10 deg. or in the beginning of June and December, 

 in the first case to the west, and in the second to the east ; I 

 should conceive that in such observations, a new projection of 

 the sun's disc, axis, Sec. is highly necessary for at least every 

 week or ten days, as in that time there will be a sensible differ- 

 ence in the inclination. I also conceive, that as we cannot al- 

 ways be sure of an observation when he is on the meridian, a te- 

 lescope mounted on a poUr axis will be found a proper instru- 

 ment, as by following the sun's motion in the parallel he describes 

 on any particular day, the projection will, if rightly constructed, 

 be correct at any distance from the meridian, provided the motions 

 of the instrument are good, and it is exceedingly well mounted. 

 I may also further observe, that during the period of observations 

 in question from September to November, the sun's axis de- 

 clined from about deg. 10 min. W. to about 3 deg. 45 min. E. 

 supposing him upon the vieridian, instead of being constantly 

 S deg. westward, as stated Ijy Mr. M. ; and consequently those 

 observations are absolvtely wrong, if the tables are right, as to 

 the position of the axis and direction of the spots. 



I recommend to such as speculate upon these curious phaeno- 

 niena, after observing the sun's image upon paper for the pur- 

 pose of tracing the places and directions of the spots, to exa- 

 mine them by means of a dark glass applied to their telescope, 

 before they describe their shape, as I am convinced from long 

 and repeated observations, tliat their true appearance is much 

 better seen that way than the other. They would also do well 

 to remember, that a telescoj)e which produces erect vision, when 

 looking in it, inverts the images when received upon paper, and 

 that if inverted vision is produced by looking in the instrument, 

 erect objects will be seen upon the paper, but the right and left 

 edges of an object will change places, when the observer's back 

 is turned upon it, as it must be in such observations. 



I prefer a dark glass, or, what is much better, a green and a 

 red one united in one cell, because the penumbra or lighter side 

 of a spot is seen perfectly defined, and smaller spots which are 

 nearly invisible bv the other method, may be distinctly seen and 

 counted if necessary by this. Indeed in a favourable atmosphere 

 with no larger instrument than Mr. M.'s, and a very moderate 

 power, I have not only done so, but have seen also the brighter 

 spots, or faeculae, or bright lumps, behind which the dark ones 

 seem to lie when near the limb, as well as the innumerable small 

 dark jjoints which cover the whole face of the sun, when by re- 

 ceiving the image on paper I could not see them at all. 



The conversion of dark spots containing midei, into bright 



ones 



