ACCESSORY APPARATUS 



source of liirht. When, however, a Dingle point of light in the 

 iixis is the source, tin- condenser will lie much move sensitive, ami 

 a Lower value tor tlu- aplanatie 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 

 tl,,. tal.le'a practical rather than a theoretical and probably truer 



result. 



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

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

 Held 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 number have been superseded by high-class condensers.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-LEXS and the PARABOLOID, although they are only 

 serviceable for very low [lowers, 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 tives, and for use with higher powers they are superseded by the 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 nil' the central rays for the pin-pose of obtaining a dark 

 ground upon \\hich the illuminated object lies. Its use is very 

 lienelicial in lou power work. Large insect preparations are pro- 

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

 when the moderate powers are Id-ought 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 Mr. Shadbolt. ingenious 

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

 It consists of a paraboloid of ^lass that relied < to its focus the rays 

 which fall upon its internal surface. A diagrammatic section of 

 t his itist rumen! . shou ini: t he course of t he rays through it, is given 

 in liii. 'J.V.l. t he shaded portion representing the paraboloid. 1 The 



A |>;ii-;il><>li<- 1 1 1 n 1 1 1 M 1.1 1 . .r was lir-l ilc\iscd liy Mr. \\Vnliaiii, \vln>, however, 

 I a si IMT >|M-riilnni tui lln |nir|>oM'. AKont tlic ^amc time Mr. Shadbolt 



- I'm- tin- -;llllr pill |M'-.i' iSft- 1 Trillin. Micro. 



vol. iii. L852, pp. 85, \'.\'1\. Tin- tun |n-iin-i|ili'S arc combined in the ;jl.is- 



