DIAPHRAGMS 297 



finder, render it extremely unhandy when using an immersion objec- 

 tive, all the more so if the condenser happens to be immersed as 

 well. 



If the Maltwood finders are made alike, they are then, of course, 

 interchangeable. 



Diaphragms. There are three kinds of diaphragms in use. 

 First, the commonest form is that of a rotating disc of several aper- 

 tures graduated as to size. Secondly, a series of separate small 

 discs of metal, with a single central aperture, which fits in a suitable 

 carrier. Thirdly, there is what is known as the * iris ' diaphragm, 

 which is shown in fig. 238. Upwards of 30 years ago it was applied 

 to the microscope by Beck ; it has since been brought to great per- 

 fection, some being made with as many as sixteen leaves ; all makers 

 now provide them. In whatever form the diaphragm may be which 

 is for use with the mirror, it ite important that it should not be 

 placed too near the object, as then" its position lies so near the apex 

 of the cone of illumination 

 that it will not cut it unless 

 the hole be exceedingly small. 

 A very small diaphragm aper- 

 ture is objectionable, as it is 

 liable to introduce diflractional 

 effects. Therefore it is better 

 to use a larger aperture 

 further away from the stage 

 than a pin-hole- near the 

 stage. When a diaphragm is 

 used in connection with a 

 condenser, it should be placed 

 just behind the back lens, and 

 never above the front lens. 

 Calotte diaphragms placed 

 close under the stage, and FIG. 238. Zeiss's iris diaphragm, 

 which have been much in use 



lately, both here and on the Continent, are a mistake for critical 

 work. 1 



A very good w T ay of cutting down a cone from a mirror is to 

 have the diaphragm fitted in the sub-stage, so that it can be made 

 to advance or recede from the object. The advantage thus gained 

 is that one aperture is made to do the duty of several. It also 

 permits of careful adjustment. 



The iris diaphragms are so comparatively inexpensive, that they 

 have superseded for general work and ordinary purposes all others ; 

 but whatever diaphragm is used it should work easily. Iris dia- 

 phragms work sometimes so stiffly that the microscope may be moved 

 before the diaphragm. So, too, with the diaphragm wheels ; some 

 require a pair of pliers before they can be rotated. This is easily 

 accounted for when we examine the way in which they are fixed. The 

 usual method is to screw the wheel to the under side of the metal stage. 

 Now, if there are neither washers nor a shoulder to the screw, it is 

 1 Quekett, Micro. Journ. vol. iv. p. 121 et seq. 



