932 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
for this setting of the particular diaphragm ; on illuminating as usual by means of the 
lantern, light of the desired wave-lengths will emerge from the slits. 
The wide-angle polariscope alluded to in the last section is of course employed as 
observing instrument, in order that the whole figure may be visible in the field at 
once. Having selected the diaphragm to be used, and placed it in position as above 
described, and set the circle to the reading recorded for the diaphragm, the difPusing- 
tube is attached in front of the slit-frame and moved as far from the latter as is 
allowed by the length of the rod, and the ground-glass screen is placed right at the 
end furthest from the slit. A cylindrical lens of short focus, carried on a convenient 
stand, is then introduced between the diffusing-tube and the slit-frame, its plane side 
towards the latter, in such a position that the two or more lines of - light are directed 
upon the same space in the centre of the ground-glass screen, where they are well 
Fig. 6. 
mixed and diffused. It is quite easy in this way to produce a patch of white light 
upon the screen, by employing a diaphragm pierced by two slits of the necessary 
apertures, through which yellow and blue rays of the requisite wave-lengths are 
transmitted and afterwards properly blended by the cylindrical lens. Upon bringing 
the polariscope close up to the difiusing-screen the field is brilliantJy illuminated with 
light of the colour produced by the admixture of the pure colours emanating from the 
two or more slits, and if a section of a crystal exhibiting the phenomenon of crossed 
axial planes is introduced between the converging-lens systems the composite inter¬ 
ference-figure will be observed. The arrangement is represented in fig. 6. 
