102 Smith^ on a Slide holder for the Microscope. 
heavier forming the stage plate ; further description seems 
scarcely needed. With many objects it is absolutely necessary 
to have them in a certain position with respect to the light to 
B evolving Slide-holder and Selenite Stage. 
see them at all^ and with many more a slight change in 
position renders them far more distinct, and I am led to think 
that in such cases this vslide-holder will be found both con- 
venient and useful from the ease with which a slide placed 
upon it can be completely rotated ; it might also be found 
useful in some cases of examining transparent objects by 
oblique lights and the hole in the lower plate might be made 
oblong, so as not to interfere with the oblique rays from the 
mirror. 
A second and more general use to which this slide-holder 
might be applied is that of a selenite-holder for use with the 
polarizing prisms. In the ^Microscopical Journal' for July, 
I860, pp. 203-4, I described a simple form of selenite stage 
having for its object a means of removing and replacing again 
the various selenites without disturbing the slide under exami- 
nation or requiring to alter the focus of the microscope. In 
the revolving stage plate I now bring under the notice of 
the Society, it will I think be seen that by means of the in- 
tervening space between the lower and secondary plates, as 
shown in a, drawing b, the selenite plates can be slipped in and 
out without disturbing the object; and in the case of this 
holder there is the additional advantage obtained of having 
the object itself re volvable, while the polarizing prisms and the 
selenites remain in a fixed position with regard to each other 
— an arrangement which brings out some remarkable effects. 
It will also be apparent that when this secondary stage is 
used with a microscope that has a revolving stage plate of 
its own, the selenite plate and object under examination may 
be made to rotate in the same or in an opposite direction to 
one another; while the polarizing prisms remain fixed. 
