The illuminator (Fig. 4) consists of a large size concentric focus- 
sing dark ground illuminator in which the optical parts have a central 
aperture into which is fitted a quartz condenser. Both these systems 
can be simultaneously focussed upon the object, and by blocking out the 
appropriate apertures at the back of the condenser, either dark ground 
illumination with visible light, or ultra-violet illumination with trans- 
mitted light is employed according to which source of illumination is 
being used. For dark ground illumination with ultra-violet light a special 
type of illuminator devised by Mr. Smiles is supplied which gives a power- 
ful illumination and with which short exposures can be given. 
Ht) Fig. 4 
Duplex dark ground (visual) : ‘ 
and transmitted and U.V. objectives in terms of wavelengths of light. A mirror fitting (aa, Fig. 3) 
An interferometer is supphed to measure the movement of the 
Sone ee is provided which can be fitted to the objective bracket (¢). This 
carries a flat mirror which can be tipped by two adjusting screws (ab). Another fitting (ac) is fixed 
to the body of the focussing unit (k). This includes a plane plate of glass (ad), a thin glass reflec- 
tor and an observing microscope (ah). A small mirror (ae) receives light from the mirror (af, Fig. 
1) of the mercury lamp and reflects this down on to the thin glass reflector, which in turn reflects 
it upon the interferometer plates. These are moved close together by means of the rack and pinion 
(ag) and the first interferometer mirror is adjusted parallel to the second by means of the two 
screws (ab). The observing microscope (ah) is then swung round to a convenient position and the 
shift of the interference bands can be observed when the fine adjustment is actuated. 
(C) The optical bench (C, Fig. 1) consists of a triangular section bar rigidly fixed to the main 
frame (A) at right angles to the axis of the microscope (B). Three fittings slide along this bar. 
The first carries a vertical rod, upon which is fitted the quartz mercury lamp. The second carries 
a condensing lens and a rack to carry any standard size colour filters. The third carries a table 
upon which is the swinging reflecting prism. All these fittings have vertical adjustments so that 
the apparatus which they carry can be accurately adjusted with the axis of the microscope (B). 
The swinging prism is so adjusted that when it is swung against one stop it reflects the light from 
the mercury lamp along the optic axis of the microscope (see Fig. 1) and when it is swung against 
the other stop, the prism is entirely removed, thus allowing an unobstructed path for the ultra- 
violet light from the optical bench (D). Full adjustments in all directions are provided. 
(D) The optical bench (D, Fig. 1) has a triangular bar similar to (C) but it is mounted so as 
to swing on a vertical axis (an, Fig. 1) which is in the same vertical plane as the optic axis of the 
microscope (B). It carries an adjustable spark gap, a fitting with a quartz collimating lens and 
a prism table. This table carries two quartz 60° prisms, one left handed and the other right handed 
in adjustable mounts arranged in such a way that the last surface through which the ultra-violet 
light passes is exactly over the swinging axis (am). These prisms are first adjusted for minimum 
deviation and then the whole optical bench is rotated round the axis (am) until the required line 
in the ultra-violet spectrum has been selected and directed along the optic axis of the microscope (B). 
If required, the bench (D) can be swung on a horizontal axis by unscrewing the lock nut and 
rotating the axis screw (an) which tips the bench in the required manner. As in the case of 
bench (C) the fittings are capable of vertical adjustment and can be slid along the triangular bar 
and clamped in position. This bench has been designed for use with a line in the spark spectrum 
70 
