length of a thick mirror 119 
_ of its own inverted image; the coincidence is tested by the parallax 
method, a magnifying lens being used. For accurate work the 
pin should be sharp and should be firmly supported. The distance 
of the pin’s tip from the vertex A is then measured. An ad- 
justable distance piece may be used or a short rod of known 
length may be attached to a steel scale so that its axis coincides 
with one edge of the scale. The ends of the rod should be rounded. 
_ We could, of course, use any point P and its conjugate Q to give 
us a value of f through (3), but it is convenient to take the vertex 
A (Fig. 1), im which the axis cuts the first surface, as one con- 
jugate point. Some grains of lycopodium are placed on the 
surface; let one grain be at A. The system will form an image 
of this grain at some point B. The distance AB is measured 
by a microscope attached to a sliding carriage (Fig. 2), the axis 
of the microscope being parallel to the direction of motion of the 
carriage. A vertical steel rod is fitted to the carriage and the 
“microscope is attached to the rod by a suitable clamp; Pye’s 
“Tdeal” clamp is convenient. The microscope is first focused 
on A and the caryiage is then moved until B, the image of A, is 
seen sharply in focus through the microscope. The distance AB 
is equal to the distance through which the carriage has been 
moved. 
The microscope should be fitted with cross-wires or with 
a micrometer scale and the absence of parallax between the 
image of the lycopodium and the wire or scale is used as a test of 
correct adjustment. The distance between the object glass and 
the eyepiece of the microscope when B is observed must, of course, 
be the same as when A is observed. 
Care must be taken to distinguish between A and its image. 
This may be done by holding an object such as a pointed piece of 
paper against the surface and then viewing the image with both 
eyes open. ‘The binocular vision will at once decide whether B is 
in front of A or behind it. The one-eyed may hold a piece of 
paper in contact with the surface and may then focus the micro- 
scope on the surface of the paper. _ If the paper is large enough to 
9—2 
