146 Mr Waddell on a New Optical Instrument . 
object-glasses of the instrument is attached to it, and that by 
removing the other object-glass from the inner end of the hori- 
zontal tube in front of the instrument, and placing it in the 
wider part of a conical mouth-piece of brass, which screws into 
the object-end of said vertical tube, as represented under Fig. 3. 
both object-glasses are brought near to each other, and it again 
becomes an astronomical telescope, with a small magnifying 
power. If to this conical mouth-piece of brass be affixed one 
of the double right-angled triangular prisms of one piece of 
glass, with its two reflecting surfaces placed at right angles, con- 
structed by me some time since, and drawn and described under 
the article Microscope of the Edinburgh Encyclopaedia before 
alluded to, then, by the use of a microscope placed at the field- 
bar as before described, it is again fitted for the purposes of a 
camera obscura, when placed in a vertical position on a portable 
stand, also represented under Fig. 3. If this instrument, in- 
stead of the vertical position it here appears in, be placed in a 
horizontal one, and the double prism removed from the ob- 
ject, and affixed at the eye-end of the instrument, then it be- 
comes fitted for the purposes of a camera lucida, because the 
said double prism is left uncovered, both above and below, as 
in the former case described. And if to the extremity of the 
said conical mouth-piece be affixed a smooth tube, with addi- 
tional magnifiers as before described, then the instrument, if so 
required, becomes fitted for the purposes of a compound micro- 
scope. 
From the various distances at which the glasses of the instru- 
ment first described are placed, it is not easy to ascertain its 
magnifying power as a microscope, by the common rule of cal- 
culation. But by the addition of a compound magnifier of half 
an inch focus to its glasses, as an astronomical telescope, I have 
found the apparent magnified surface of the field of view, when 
at the same distance from the eye that the image of the object 
painted within the instrument was from the object itself*, and 
when the magnified appearance of the object filled the whole of 
that field, to be, when compared with the real size of the object 
viewed, as 1600 to 1. Alex. Waddf.ll. 
Hermitage Hill, \ 
7th May 1821. J 
To Dr Brewster . 
