202 OPTICAL PROJECTION 



a slide f -inch diameter, is arranged to be used in the barrel of 

 the instrument ; the analyser is a square-ended Prazmowski 

 prism. It is needless to describe the arrangements in detail. In 

 the most complete form of the apparatus, the analyser is fitted 

 with a Klein's quartz plate, and a quarter-wave plate ; and a 

 system of convergent lenses has also been arranged, somewhat 

 similar to that described on p. 843, which will exhibit the rings 

 in crystals, taking in both systems up to the angle of selenite. 

 The ordinary microscopic crystallisation slides make mag- 

 nificent displays in this form of the instrument. The loss of 

 illumination being very great with polarised light, about the 

 utmost limit of the power of the instrument, with the 0. H. 

 light, will be to exhibit the rotating cross in one or two of 

 the larger starches, such as Tous-le-mois. The arc light will 

 of course go considerably farther. 



101. Slides and Objects. Organs and parts of insects 

 make excellent objects, if mounted flat ; those mounted ' with- 

 out pressure ' fail, being both opaque, and impossible to focus 

 in one plane. The essentials are flatness in the preparation, 

 and neither too much nor too little colour. Experience will 

 soon teach the proper amount of the latter, and a suitable 

 slide can always be found amongst any decent stock. 



Transparent objects of all kinds are the most difficult, and 

 histological slides especially so, unless stained well and to the 

 proper degree. Such sections must also be thin, and it is not 

 easy to purchase such, stained so as to give the best results. 

 Where good ' differential ' staining can be got in a thin section, 

 the object can generally be exhibited under the greatest 

 illumination, and the positive colour in the structure shows 

 distinctly ; but with a clear preparation the substage condenser 

 must either be racked back from the position that gives most 

 light, or, what is better, if an iris diaphragm is fitted to it, 

 this must be contracted so as to reduce the cone of light, and 

 thereby get more distinctness in the phantom-like detail. To 

 take an instance : a section of skin with the sudorific glands 



