6 4 



PRACTICAL MICROSCOPY. 



York, supplies a new form of iris diaphragm to all his 

 " working " microscopes. 



It is now universally appreciated that the diaphragm must 

 be placed immediately beneath the object, in order to get 

 the best results. This has been attained by the " calotte " 



diaphragms now made by 

 Messrs. Zeiss, Swift, and 

 others. This form is 

 placed over the achro- 

 matic condenser, on a 

 level with the stage, as 

 shown in Fig. 40 ; but the 

 simplest way to effect 

 this is by using a thin 

 plate of aluminium 3-inch 

 by I -inch, with the desired 

 aperture pierced in its 

 centre, and furnished 

 with two spring clips, so 



that the slide may be placed upon it in any required posi- 

 tion, such as has been used by the author for the past 

 seven years. 



There is another method of excluding the excess of 

 extraneous light, used by the author since 1873 with very 

 good results viz. that of slipping over the objective a 

 small perforated cardboard and aluminium nozzle, black- 

 ened inside, the central hole in the front being just a 

 trifle less than suffices to admit the rays passing from the 

 field of vision. 



For many purposes it is necessary to concentrate the 

 light to a greater extent than can be done with a mirror 

 alone, and in such instances an achromatic condenser is 

 usually employed. This may be improvised from an 

 ordinary objective made to screw into an adapter below 



FIG. 40. 



