C 1 7 1 ] 
the paper. I alfo, before I took off the paper, 
mark’d one end of each fruftum in the edge at m 
and ?i y that if they fell afunder, they might be ce- 
mented together again the fame way. This model 
(made of a fpedtacle-glafs about 12 or 13 inches fo- 
cus) gave me encouragement to try the following 
one, which I thought better. 
Fig. 4. 
I made my fecond model of the two middle fruf- 
tums me dhwp, me dbvg, of the lens Fig. 2. by 
cementing their edges, hwp, bvg y together, as they 
are placed in the prefent Fig. 4. fo the pole e of each 
part muff confequentJy be half an inch (fuppofing its 
focal length is about 104 inches) from the middle 
where e flood in Fig. 2. viz. the pole of one fruf- 
tum where v, and of the other where w now ffands. 
I left open at each pole a femicircular aperture r wq y 
s v t, about two thirds of an inch diameter, and co- 
ver’d all the reft of the circle axl kz 0, to which I 
had cut it fit for the tube. The focus of the lens I 
made it of was about 3 feet. 
Note, The rays of red light in the two folar images 
will be next to one another in both thefe models, 
which, I take it, will render the fun’s difk more 
eafy to be obferved than the violet ones. This I 
mention, becaufe the glaffes in thefe two forts are 
fomewhat prifmatical, but moftly thofe of my iirff 
model, which could therefore bear no great charge. 
Alfo the fruftum on the right-hand of my firff mo- 
del renders the folar image at the focus on the left, 
and that on the left-hand renders it on the right : 
but it is not fo with the fecond model, or with the 
next contrivance, which is the beft, if well made. 
Y 2 Fig. 
