Smith, on a Dissecting Microscope. 13 
found ']aeqessary_, an advantage too obvious in many instances 
to need further comment. As pieces of glass of several widths 
would be required to suit objects of diflPerent sizes^ thus ne- 
cessitating the employment of several troughs fitted up in 
this manner, it has occurred to me, that in case this might 
form a ground of objection with some to the plan, the 
same purpose might be answered by simply fitting slips 
of glass of the sizes most generally useful into flat pieces of 
gutta percha, which would go into any of the ordinary glass- 
bottomed troughs, and thus easily be substituted the one for 
the other. 
No. 3. 
It now only remains for me to notice the arrangement 
figured in the above drawing, which shows a sectional view of 
the stage of the microscope, under which is held, by two 
catches, a large trough, about an inch and a half in depth, 
and having a glass bottom ; a piece of sufficient size is also 
cut out of the stage, and a moveable plate of glass, or metal, 
put in its place (as shown in the drawing), which is of course 
lifted off when the trough is used ; another, and perhaps 
more effectual, way of getting at it would be, by making a 
portion of the stage nearly equal in length and breadth to 
the trough to slide in a groove, like the lid of a small box, 
which could just be pulled out when required. By this method, 
I think that many dissections, that in the ordinary way would 
be carried on apart from the microscope, might be made on 
it ; thus allowing the hand-rests, mirror, condenser, and other 
appurtenances of the instrument, to be made use of more 
advantageously and with greater ease to the dissector than if 
a trough of this size were placed on the top of the stage. In 
order to make this arrangement as complete as possible, I 
further propose to fit a piece of gutta percha into the bottom 
of the trough, which could be taken out when wanted (and 
thus make it serve the place of two troughs) ; and also to make 
a smaU hole in it (fitted with a plug), so as to allow of the 
water being drained off when necessary into a vessel held 
below, without having to remove, or otherwise disturb the 
object under dissection in any way, which might thus, if 
