Accessory Apparatus 111 



nosepiece ; no special adaptation is required. When the 

 revolving nosepiece is screwed home, the objectives not in 

 use must point towards the middle of the front of the stage, 

 otherwise in rotating the objectives they are, with low 

 powers, apt to foul the rack-work bar of the microscope. 



The Nosepiece Iris Diaphragm, or Davis's Shutter. 



This is a very compactly made iris diaphragm, which is 

 placed between the nosepiece of the microscope and the 

 objective. Its special function 

 is to enable the aperture of an 

 objective to be decreased, so 

 that it may be used with dark- 

 ground illumination, or to in- 

 crease penetration when ex- 

 amining objects having several 

 planes. For photographing 

 opaque objects with low powers Fm< 43< _ DAVIS > S SHUTTER. 

 it enables the appearance of a 



small round object, such as a moth's egg, to be taken quite 

 sharply. We have been shown photographs that were 

 taken by a gentleman in America with a low-power objec- 

 tive, having an iris diaphragm fitted between the combina- 

 tions of lenses, by the limitation of the aperture of which 

 considerable depth of focus was obtained. The writer has 

 not heard of this scheme having been tried in England. 



The Davis's shutter is furthermore very useful for examin- 

 ing and experimenting with the diffraction spectra seen on 

 looking down the microscope tube at the back of the objec- 

 tive, when the eyepiece is removed and a striated object is 

 being examined. 



This iris diaphragm must have its aperture perfectly 

 central and the threads quite true. The aperture of the 

 iris, when completely opened, should be as great as it is 

 possible for the inside of the mount of an objective to be, 

 but the box of the iris diaphragm must not be so large as to 



