Improvements in the Micro-spectroscojye. By F. II. Ward. 329 
this much easier than they might imagine, and that no one 
accustomed to the delicate manipulation of microscopic work 
would find the slightest inconvenience from this cause. 
For micrometric purposes, however, I have adoped the method 
which has been for some time in use, i. e. the image of a photograph 
scale reflected from the upper surface of the top prism. This 
image passes through two collimating lenses to render the rays 
parallel, and can be focussed by a sliding movement of the lenses 
in an inner tube by means of two pins passing through slots : 
I have had the head of one of the pins made to screw on, so that 
when the right position is obtained, a slight turn of the head 
renders it immovable by clamping the inner tube. The photo- 
graph image may be moved in three directions ; it may be placed 
across the top, bottom, or middle of the spectrum by rotating the 
cap, which carries the arm ; and it may be rendered parallel to the 
spectrum by rotating the arm on its own axis — a clamp is furnished 
for fixing it in the latter position ; thirdly, the image may be made 
to traverse the spectrum so as to get any division to correspond 
accurately with any band by a micrometer screw divided into 
lOOths of an inch, the head of the screw being also divided into 
tooths gives a reading with a fixed index of 10,000ths. The 
scale reads in the same direction as the wave-lengths, that is, 
the highest numbers on the scale are found at that end of the 
spectrum which has the greatest number of wave-lengths. 
For the comparison prism I have adopted one which has been 
in use a short time for astronomical purposes, that is, a right- 
angled prism with a slot ground in it ; through this slot the light 
passes from the object on the stage of the microscope, whilst the 
portions of the prism on either side the slot transmit the light from 
the object on the comparison stage of the spectroscope. The effect 
of this is that the spectrum is divided into three parts, the upper 
and lower of which are the same, if the aperture in the comparison 
stage is covered by one object ; but by a proper arrangement of two 
objects on the comparison stage, it is quite easy to display three 
different spectra at the same time ; I do not, however, think that 
there is much practical advantage to be gained by so using three 
spectra. But I think in the case of two spectra but slightly 
different from each other, in the position of their lines, two compa- 
risons are better than one. It is as though you made two obser- 
vations at the same time. 
The face of the prism from which the reflection is obtained, is 
covered externally with a plate of very thin sheet copper, the 
surface of which is blackened. The comparison prism is moved 
into or withdrawn from position by the fitting to which it is 
attached, sliding in brass dovetail grooves ; this, 1 think, is more 
steady than the usual method of mounting it on a cylindrical rod ; 
VOL. I. 2 A 
