2 Slack, on a Diaphragm Eye-piece for the Microscope. 
these milled heads moves one shutter, which is capable of 
closing half the field. By combining the movements of the 
four shutters in various ways, it is easy to form a series of 
symmetrical apertures bounded by straight lines, and of any 
dimensions required. Unsymmetrical apertures can also be 
formed, but they are usually very disagreeable to the eye, 
and ought therefore to be avoided. 
Astronomers have long been acquainted with the advantage 
of restricting their field when looking at luminous objects of 
lerge size, and the solar eye-piece of Mr. Dawes is a contri- 
vance having this end view ; but so far as the writer is aware, 
microscopists have not been in possession of any analogous 
instrument. 
The only objection to the general application of this 
diaphragm stop to A eye-pieces, in the form now shown, is 
that it slightly diminishes the full size of the field. This 
arises from the writer having required that the shutters should 
meet when drawn out to their full extent. To accomplish 
this the shutters are rather broader than the ordinary stop. 
If kept within the dimensions of that stop they would pro- 
bably do all that is required, as experiment shows that the eye 
is incommoded when the field is very small. 
The diaphragm stop is most needed in the examination of 
small objects sufficiently opaque to require a very intense 
light for their display by transparent illumination ; but other 
objects, such as diatoms and butterfly scales, are improved in 
appearance, and have their markings made more striking 
when the extent of the luminous margin surrounding them is 
nicely adjusted to their special requirements. 
The diaphragm stop may also be used as an indicator, as 
it affords a ready means of isolating one out of many objects 
that may be on a slide. 
The writer believes that it will be found of much use in 
protecting the eyes of microscopists, which are more liable to 
injury from excess of light than from any other cause (bad 
glasses excepted) incidental to their pursuits. 
