298 ACCESSORY APPARATUS 



more than probable that when the diaphragm is rotated it will 

 screw up and jam. The purchaser may easily observe a matter of 

 this kind. Cylinder diaphragms, which were invented in 1832 by 

 C. Yarley, are much used on the Continent ; they are also often made 

 into iris forms. Also diaphragms with a very minute circular hole 

 in the line of the optical axis are largely used just behind the 

 object -slip. These are employed with the mirror only (without 

 condenser) and with daylight alone. The object of this method 

 of illumination being to render very translucent objects visible by 

 increasing the size of the black diffraction bands at their edges, it 

 is, as before stated, of no use for critical work. 



Condensers for Sub-stage Illumination. 1 This condenser is an 

 absolutely indispensable part of a complete microscope. Its value 

 cannot be overrated, for the ability of the best lenses to do their 

 best work, even in the most skilful hands, is determined by it. 

 Perfection in the corrections of object-glasses is indispensable ; but 

 those who suppose and affirm that this is all that we need that the 

 objective is the microscope cannot understand the nature of modern 

 critical work. The importance of it could not have been realised in 

 the sense in which we know it in the earlier dates of the history of 

 the instrument ; but at as early a period as 1691 we pointed out 

 (p. 134) that a drawing of Bonanni's horizontal microscope showed 

 the presence of a condenser. It is, in fact, of some interest to note 

 how our modern condensers gradually arose. 



The microscope that amongst the older forms (1694) appears 

 most efficient and suited for the examination of objects by trans- 

 mitted light was that of Hartsoeker (p. 134, fig. 102). It will be 

 remembered that it was furnished not only with a condenser, but with 

 a focussing arrangement to be used with it, which was not in any 

 way affected by a change of focus in the object. This is a feature 

 which, although not then important, is of the utmost importance now. 



In the correction of dispersion in the lenses employed in the 

 dioptric form of microscope so much difficulty was experienced that 

 several efforts were made to produce catoptric forms of the instru- 

 ment ; the most successful of these was that of Dr. Smith, of Cam- 

 bridge, in 1838 ; but this and all other forms of reflecting microscope 

 had but a brief existence, and passed for ever away. To the improve- 

 ment of simple lenses much of the earlier progress of microscopic 

 investigation is attributable ; and that known as ' Wollaston's 

 doublet,' devised in 1829, was a decided improvement in all respects. 

 It consisted of two plano-convex lenses ; but this was again improved 

 by Pritchard, who altered the lens distances and placed a diaphragm 

 between the lenses. When the object was illuminated with a con- 

 denser this formed what was the best dioptric microscope of 

 pre-achromatic times. 



Good results, within certain limits, may be obtained by means of 

 the best Pritchard doublets. With a -j^th inch the surface of a 

 strong Podura scale may be seen as a surface symmetrically scored 

 or engraved ; but the Editor has never himself been able to reveal the 



1 The word ' condenser ' throughout this work is applied to optical appliances for 

 the sub-stage ; what is known as the ' bull's-eye ' is not called a ' condenser.' 



