98 PHOTO-MICROGRAPHY 



those recently introduced by Mr. Conrady, to whom reference has already been made.^ 

 Whether this be due to the same thought in their construction, (see page 7^), which 

 he holds to so strongly in the manufacture of his lenses already mentioned, it is 

 difficiilt to say ; but anyhow they transmit a solid aplanatic cone of great purity, 

 nearly— not exactly— of the same diameter as their aperture, and how this can 

 best be ascertained and proved must be the next subject. 



Let us consider the following. It is desired to ascertain the size of the largest 

 aplanatic cone of a given condenser, whose N. A., we will say, is stated to be ro. 

 Fix it in the usual position on the substage, and place on the nose-piece of the 

 microscope first an objective of N. A. S, and on the stage a diatom. Focus it with 

 the objective, using as an illuminant the edge of the flame, and rack the condenser 

 up and down until this image of the flame is seen across the field with the diatom 

 lying in its centre. This is obtaining what is technically called critical light, and the 

 resulting image is called " The critical image." Shift the diatom just out of the field 

 of view, still leaving a portion of the slip and its cover-glass m situ. Remove now 

 the eye-piece, and look down the tube of the microscope. One ought to see the back 

 lens of the objective full of light, because the aplanatic cone of the condenser should 

 be greater than that of an objective 0*6, such as we are supposed to be here using. 

 Return now the eye-piece and remove the objective, substituting one of 0-95 N. A. ; 

 again focus the diatom, and again obtain critical light by focussing the condenser on 

 the diatom until the edge of the flame is seen across the field. Once more shift the 

 diatom out of the field, and look down the tube as before. The back lens should be 

 quite evenly filled if the aplanatic cone equals the numerical aperture. Then close 

 the iris diaphragm until its edge is just seen, and carefully note the exact size of the 

 opening with a pair of compasses. Now remove the 0-95 and place in its stead an 

 objective of i -40 N. A. Treat as before, with respect toTocussiiig and obtaining critical 

 light, and look down the draw tube. Only the centre two-thirds of the back lens is now 

 seen full of light, and the shghtest touch of the condenser upwards so as to try and 



* Whilst these pages are passing through the press, our attention has been called to two somewhat new substage 

 condensers, one made by by Messrs. Watson and Son, the other by K. and J. Beck, of Cornhill. That by Watson is called 

 the Parachromatic Substage Condenser. We find its N. A. is i -o, and that its aplanatic cone exceeds -90 by actual experiment. 

 Its focus is f inch when used for high powers. Besides this it is so constructed that the top lens is removable, which 

 lowers the power to about t%- inch, when it can be employed for objectives of lower angle. We cannot adequately express 

 our satisfaction with the performance of this piece of apparatus ; it appears to be well corrected, and we think it 

 has a great future before it, and we feel bound to offer the manufacturers our heartiest congratulations at being able to 

 make as perfect an article at so low a price. It is so nearly opo-chromatic that it is not a little doubtful if it had been 

 so-called we could have detected it to be otherwise. Its mount is also exceedingly neat, small, and well made, and it has 

 a large working distance, and when used as an objective defines most admirably. 



The arrangement by Beck has justly earned considerable repute. We tested it some little time ago, and found it proved 

 a very admirable piece of work. 



