352 
ME. WAEEEN DE LA EEE ON THE 
reflector drawn out so as to place the silvered surface in the field, f is the socket- 
adapter, which screws into the telescope ; it has a slit cut into it to receive a pin fixed 
on the sliding tube, the object of which is to keep the position-lines of the eyepiece, 
when once adjusted, in their proper position ; e is the glass reflector, the half towards d 
being silvered, that towards e plain; is a covering to protect the sliding reflector; 
^ is a circle fixed to the body of the eyepiece, having one quadrant divided into nine 
spaces of 10° each ; a is an index attached to a positive eyepiece, which can be moved 
by it through an arc of 90° ; c is a graduated sun-shade, composed of a wedge of dark 
glass and another of white glass, reversed in position, so as to form, when combined, a 
parallel plate. This is held firmly in its place by means of a spring, shown in fig. 3, 
which, while it holds the shade firmly in any required position, also allows its instant 
removal at pleasure. The glass reflector e, as soon as the observer desires to use the 
silvered surface, can be drawn forward in a small fraction of a second, without disturb- 
ing any other part of the instrument. 
In the focus of the positive eyepiece was fixed a piece of parallel glass on which were 
etched several lines ; this micrometer- plate was carried round with the eyepiece when- 
ever the index, a (figs. 1 & 3), was moved. A reference to Plate VI., which contains 
a fac-simile of my hand-drawings, and also a representation of the position-lines, will 
render clear the following explanation. Four principal lines on the glass plate formed a 
tangential square calculated to enclose exactly the moon’s disk, which in fact it accom- 
plished with great precision ; four other lines surrounded the first square at the distance 
of exactly I' of arc ; and a third series formed a third square at the same distance from 
the second. Joining the angles of the squares were two diagonal fainter lines, which 
served to measure angles of position, while the several squares served to measure 
distances. The angles of the tangential square may be designated A, B, C, D. As 
M 
n 
z 
H 
I 
O 
c 
H 
Z 
soon as the axis of the telescope-stand had been adjusted in a vertical position by 
