13 and 15.

Having then drawn a line to represent the sun's ecliptical diameter inclined to the horizontal diameter as above described, and having (with this line to correspond to _ab_ in figs.
12-15) drawn in the sun's equator suitably inclined and opened out, he has the sun's actual presentation (at noon) as seen with an erecting eye-piece.

Holding his picture upside down, he has the sun's presentation as seen with an astronomical eye-piece--and, finally, looking at his picture from behind (without inverting it), he has the presentation seen when the sun is projected on the screen.

Hence, if he make a copy of this last view of his diagram upon the centre of his screen, and using a low power, bring the whole of the sun's image to coincide with the circle thus drawn (to a suitable scale) on the screen, he will at once see what is the true position of the different sun-spots.

After a little practice the construction of a suitably sized and marked circle on the screen will not occupy more than a minute or two.
[Illustration: _Fig.