168 THE MICROSCOPE. 



Podura scale. Thus, this arrangement enables an experi- 

 menter viewing a new object to see surface markings, and 

 to obtain penetration with one and the same stop ; an 

 important gain in original investigation. The slot stops 

 have been found very useful in investigating unknown 

 objects, as well as in displaying those that are known ; 

 and with the whole aperture and the two-slot stop, many 

 diatoms with double sets of lines are well brought out. 

 The microscopist will find that from 40° to 59° angle of 

 aperture will, in many cases, give the best results, when a 

 ;| or ^ objective is employed. 



An excellent condenser of very large angular aperture 

 is made by Messrs. Powell and Lealand, in which every 

 requisite modification of the illuminating pencil may be 

 produced by two revolving discs, one containing apertures 

 of various sizes, and the other various diaphragms for 

 excluding the central portion, or for admitting only 

 angular portions, of the pencil of light. These discs are 

 placed immediately below the posterior lens of the illumi- 

 nator. This method of modifying the illuminating pencil 

 was first applied in Gillett's condenser, as constructed 

 by Mr. Ross. The improved hemispherical condenser 

 lately introduced by the Rev. J. B. Reade, answers 

 its purpose remarkably well. The plane surface of the 

 hemisphere is placed upwards, and is covered by a dia- 

 phragm in which are marginal apertures, capable of adjust- 

 ment either to an interval of 90° with each other, when 

 the arrangement of the dots to be developed is quad- 

 rangular, as in P. }'homboides, or P. hippocajnptis^ or to 

 one of 60° and 120°, when they are arranged in equilateral 

 triangles, as in P. angidatum, &c. An ingenious plan of 

 illuminating minute objects, mounted in Canada balsam, 

 by reflected light, has been devised by Mr. Wenham : 

 this consists of a small truncated glass paraboloid, which 

 is temporarily attached to the under side of the slide 

 containing the object, by a little gum, oil, or fluid Canada 

 balsam. The rays internally reflected from the convex 

 surface of the paraboloid, impinge very obliquely on the 

 under-surface of the slide, are transmitted in consequence 

 of the fluid-uniting medium, and then internally reflected 

 from the upper surface of the covering glass on to the 



