NELSON'S CONDENSER 'CORRECTION COLLAR 



303 



examined when an object is illuminated by the full aperture of the 

 condenser, the edge of the flame being in focus, it will be noticed 

 that the illuminated 

 portion of the back lens 

 will be oval and pointed 

 instead of circular. 

 Also that when the 

 condenser is racked up, 

 although the exterior 

 shape of the illuminated 

 portion will become 

 more circular, two dark 

 patches will appear on 

 either side of the centre, 

 showing the operation 

 of the spherical aber- 

 ration of the condenser. 



If under these circum- FlG u*.^l*on>* correction collar to Powell's 

 Stances me lenses be apochromatic condenser. 



separated by means of 



the collar adjustment, the black spots will be closed up, and a circular 

 and evenly illuminated disc will appear. This is a distinct optical 

 gain, and will enable the observer to see more than he could 

 have seen before. Mr. Nelson made this manifest on the examina- 

 tion of a well-known diatom, Navicula major. If examined in its 

 ' principal view/ two vertical stripes will be seen running down the 

 centre of the hoop (fig. 243, a) ; these can easily be resolved into 

 striae with a J-inch objective, but the probability is that these striae 

 are not the real structure but rows of minute perforations incom- 



FIG. 243. 



FIG. 244. Watson's oil-immersion condenser. 



pletely resolved (fig. b) ; by using the condenser with the collar 

 correction these striae were resolved by means of the enlarged 

 aplanatic cone it produced, as shown in c. 



Another advantage of the correction collar is that it enables the 

 worker to determine most delicately the size of the illuminating 

 cone, and so to record it that it can be with facility exactly resumed 

 at any time (Journ. A 3 . Micro. Soc. 1895, pt. ii. p. 231-2). 



One of the most valuable condensers introduced by any maker 

 lately is an oil-immersion one by Messrs. Watson and Sons. It has 

 special claims upon the attention of those who work with high 



