ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 241 



having opened up the Nelson iris, if there are reddish fringes at the 

 sides, "the Nelson lens must be moved a little nearer to the lamp until 

 the whole field is illuminated with uniform white light, after which it is 

 well to see that its centring has not been disturbed. 



7. Adjust the opening of the S.S. iris to suit the object to be photo- 

 graphed. 



8. All that now remains to be done is to interpose such screens as 

 may be considered necessary ; to estimate the exposure, introduce pro- 

 jection ocular, and we are ready to expose the plate. 



Results. — We already know that the field is brightly, uniformly, and 

 fullv illuminated ; but if we remove the ocular and look down the tube 

 we shall see, if the sub-stage iris be opened fully, that the bright, circular, 

 illuminated area fills the whole of the lens. If we measure the bright 

 area we shall find that it represents about 0'95 N.A., which is the 

 aplanatic cone usually obtained with a I'O N.A. dry condenser and the 

 full N.A. of the objective. 



If we wish to use a larger working aperture than • 95 in the case of 

 oil-immersion objectives, then we must exchange the dry condenser for 

 an oil-immersion one, and the object must be mounted in balsam. 



But with a 3 mm. objective of 1"30 N.A., and w^ith the immersion 

 condenser, you will find that, with the collecting system in use, the full 

 N.A. of the objective is not even then utilized. On looking down the 

 tube you will see that the back combination of the lens is crossed by a 

 broad ribbon of light of a breadth equal to a N.A. of 0'95, although 

 its length is equal to a N.A. of about 1'30. This incomplete, band- 

 form of illumination is not to be recommended, especially with high 

 powers. 



If we want to find the explanation of such a result, we have only to 

 see that the sub-stage iris is sufficiently opened up to obtain the full N.A, 

 of the objective, and look at the image of the lamp focused on the back 

 of it, and we shall see that it is not broad enough to completely cover 

 the opening of the iris. To remedy this we can employ what we might 

 call — 



Modification of Normal lUuminating Method. — This consists in placing 

 a concave 7 in. spectacle lens at about 7 in. behind the substage iris. 

 This has the effect of broadening the image of the lamp filament on the 

 back of the iris so that it is capable of completely covering the opening 

 of the iris. After this lens has been centred, which is done by looking 

 through the ocular and moving the spectacle lens about until the Nelson 

 iris, which must be closed for the purpose, is right in the centre of the 

 field, the Nelson lens must be moved a little farther from the lamp, so as 

 to again give a sharp image of the filament on the back of the S.S. iris, 

 and the sub -stage condenser must be re-adjusted so as to bring the Nelson 

 iris again sharply into focus on the field. The back combination of the 

 objective will now be found to be fully illuminated. 



With medium-power objectives {\ in. and h in.) we can still use the 

 normal system of illumination ; but we must use a low-power sub-stage 

 condenser. Personally, I use the Beck dry condenser, with front removed 

 (N.A. • 40) ; and we get with that arrangement the whole field uniformly 

 illuminated, and also the full N.A. 



