ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 



291 



diverging rays) from what is often an ill-arranged* concave mirror. The 

 concave mirror is generally made too small, and of too shallow a curve. 



I now come to the second part of my subject, viz. the condenser and 

 the objective back. 



1. It is advantageous to know the maximum aperture of your 

 condenser. It can be easily measured by an Abbe's apertometer. 



2. It is of more importance, however, to know the aperture of the 

 largest cone free from spherical aberration which can be obtained from 

 a condenser. This cannot be found out by the apertometer. 



3. It is also of paramount importance to know the apertures of the 

 cones which the various diaphragms will give. These, of course, could 

 be measured by an apertometer ; but with very many forms of dia- 

 phragms it could only be accomplished with difficulty. 



I will now endeavour to give some useful hints with regard to these 

 questions. I will take No. 3 first. 



Fig. 51 exhibits the back of a dry lens of N.A. 0-5 = 60^ illumi- 

 nated by a condenser of greater aperture free from spherical aberration, 

 the condenser and flame image having been centered and focused. 



Fig. 52 shows the same objective, with a smaller diaphragm placed 

 at the back of the condenser. The edge of this diaphragm is seen just 



Fig. 51. 



Fig. 52. 



Fig. 53. 



Fig. 54. 



appearing at the margin of the objective. The aperture of the con- 

 denser, when used with this diaphragm, is therefore a shade less than 

 N.A. 0-5— say 55°. 



In a similar manner the apertures of the condenser with other 

 diaphragms may be estimated with sufficient accuracy for all practical 

 purposes. 



Now with regard to No. 52, it is necessary to have a wide-angled 

 objective ; the condenser and flame are centered and focused as before, 

 the eye-piece removed, and while the back of the objective is being 

 examined the condenser is slowly racked up. It will be noticed that a 

 point is reached when the disc of light is at its largest (fig. 53) ; on a 

 further movement of the condenser two black spots appear, one on either 

 side of the middle of the disc (fig. 54), and these increase as the con- 

 denser is further racked up. The last point before the appearance of the 

 hlack spots indicates the largest aperture of the condenser free from spherical 

 aberration, and is the limit of the condenser for critical ivorJc. Any further 

 advance of the condenser gives merely annular illumination, which, of 

 course, is to be avoided, except in the case of dark grounds with stops 

 for low powers. The extreme margin of even the best condenser is only 

 useful for giving an oblique beam with a slot." 



* The concave mirror is usually fixed to the tail-piece, with or without a crank. 

 This is thoroughly wrong in principle. A concave mirror should always be fixed to 

 a tube sliding in the tail-piece to allow of it being focused. The crank-arm is quite 

 of secondary importance. 



