0 
of Ach romatic Object- Glasses. 
the exterior tube close upon it. This tube projects to the line 
c d , which shews its termination, efh i is another tube which 
slips over c d, and carries at its end fi the circular plate gh^ 
graduated on its outer edge from zero both ways to 180° ; v is 
a vernier attached to the first outward tube Im n o. The dia- 
meter of these tubes may be about 2J inches. The end of the 
tube efli i has an end or base at e h , in which is a circular hole 
about 1 J inch in diameter, and against this there is a means of 
fixing a prism, as shewn in the figure. The tube cd is also ter- 
minated at c d with a similar end for the same purpose, but is 
made to slip out and in like a common diaphram, for the conve- 
nience of fixing the prism on the inside, in order that the in- 
terior faces of the two prisms may be parallel. 
The construction of this instrument will be better understood 
by referring to Figs. 6, 7, 8, 9, 10, where Fig. 6. is the case-tube 
fixed to the stand, with its vernier and set-screw ; Fig. 7. is the 
next tube inserted into this ; Fig. 8. is its diaphram for carry- 
ing the prism inserted into Fig. 7. ; and Fig. 9., is a short tube 
with a graduated circle, which fits over Fig. 7., and which also 
carries a prism, as seen in Fig. 5., where the several tubes are 
all in their places. 
This instrument being thus provided, we must next get a 
piece of smooth board, about 2 feet square, well blackened with 
lamp-black, across which is to be stretched a parallel strip of very 
white clean card-paper. This is to be hung up, with the card- 
paper horizontally, in a good light, with a plumb-line passing 
across it as in Fig. 10. Then set up the dispersive instrument 
in front of it, at the distance of about 6 or 8 feet, and every 
thing will be ready for observation. 
10. Method of Observing. 
1. Remove the tube and graduated circle Fig. 9., with its 
prism, which is always to be that possessing the greater disper- 
sion of the two, and turn the tube Fig. 7. about in Fig. 6. till 
the edge of the prism fixed to its end is upwards and perfectly 
horizontal, which will be known by the eye perceiving the plumb- 
line directly above the edge of the prism, and the refracted 
image of the same in the prism in one vertical line. For which 
