31,6 ACCESSORY APPARATUS 



source of light. When, however, a single point of light in the 

 axis is the source, the condenser will be much more sensitive, and 

 a lower value for the aplanatic aperture than that given in the table 

 will be obtained. But as a single point of light is seldom, if ever, 

 practically used in microscopy, it was deemed better to place in 

 the table a practical rather than a theoretical and probably truer 

 result. 



It has been stated that the best dark grounds are obtained when 

 a stop is used which is of just a sufficient size to give a suitable dark 

 field and no more. 



When such a stop has been chosen, and excellent results are ob- 

 tained with, say, balsam-mounted objects, if, in the place of this, 

 living animalcules in water be examined, it will probably be found 

 that a dark field can no longer be obtained. 



For animalcules in water and ; pond life ' generally a stop larger 

 than that employed for ordinary objects will be necessary. 



Other Illuminators. In the course of the history of the micro- 

 scope a large number of special pieces of apparatus have been devised 

 for the purpose of accomplishing some real or supposed end in illumi- 

 nation. Many of these have proved wholly impracticable and had a 

 mere ephemeral existence ; many more never accomplished the end 

 for which they were supposed to be constructed ; and a still larger 

 number have been superseded by high-class condensers. 



The great majority of these illuminators were devised for the 

 production of oblique light. In the sense in which it was employed 

 a few years ago, it is rendered needless by condensers of great aper- 

 ture. All the obliquity at present needed can be obtained with good 

 condensers. 



To give completeness to this part of our subject it is needful to 

 refer to the SPOT-LENS and the PARABOLOID, although they are only 

 serviceable for very low powers, such as 3-inch to H-inch objec- 

 tives, and for use with higher powers they are superseded by the 

 condenser. 



A spot lens is a condenser with a permanent axial stop fixed in 

 it to cut off the central rays for the purpose of obtaining a dark 

 ground upon which the illuminated object lies. Its use is very 

 beneficial in low-power work. Large insect preparations are pro- 

 bably better shown with this device than with any condenser, but 

 when the moderate powers are brought into operation the condenser 

 at once makes manifest its superior qualities. 



The paraboloid, or parabolic illuminator, as devised by Mr. 

 Wenham, and subsequently improved by Mi'. Shadbolt, ingenious 

 and beautiful instrument as it is, comes under the same category. 

 It consists of a paraboloid of glass that reflects to its focus the rays 

 which fall upon its internal surface. A diagrammatic section of 

 this instrument, showing the course of the rays through it, is given 

 in fig. 259, the shaded portion representing the paraboloid. 1 The 



1 A parabolic illuminator was first devised by Mr. Weiiham, who, however, 

 employed a silver speculum for the purpose. About the same time Mr. Shadbolt 

 devised an annular condenser of glass for the same purpose (see Trans. Micro. 

 Soc. ser. i. vol. iii. 1852, pp. 85, 132). The two principles are combined in the glass 

 paraboloid. 



