58 PEACTICAL PHOTO-MICKOGEAPHY 



Keference to the figure will clearly indicate the path of the rays, 

 and it will be seen that the object lies at a point where the whole 

 of the light is condensed which the paraboloid transmits. This 

 apparatus is simple in use, providing that certain essential 

 points are carefully attended to. Concentration must be 

 carefully carried out, and Messrs. Ziess recommend that the 

 objective should be supported on a centreing nose-piece in 

 addition to the ordinary sub-stage centreing arrangement of 

 the condenser. If, however, the sub -stage itself is carefully 

 centred, it would not appear that the objective requires any 

 such adjustment ; but in some microscopes the centreing sub- 

 stage will not carry the paraboloid. The thickness of the object- 

 slide must be exactly that for which the condenser is designed, 

 and must not vary more than 1 mm. The thickness of the slide 

 should be between 1/3 mm. and 1/4 mm., and the thickness of the 

 cover-glass should also be exactly that for which the objective 

 is corrected. As in other dark-ground methods, when used 

 with high powers exact correction for thickness of cover-glass 

 must be carefully effected, either by collar adjustment in the 

 objective itself, or by alteration of optical tube-length. The 

 same precautions as to the elimination of any extraneous object 

 or defects in the object-slide or cover-glass must be attended to 

 here as in other dark-ground methods. This illuminator, having 

 greater efficiency than any yet described, is more sensitive to 

 any disturbing influence than any method in which the ordi- 

 nary sub-stage condenser is used. A strong source of light is 

 required ; but for visual work a Welsbach incandescent gas- 

 light is sufficient. For photographic work a more powerful 

 illuminant is necessary ; but unless instantaneous exposures 

 are required, a one-ampere Nernst lamp, or some similar 

 illuminant, will answer most purposes. 



A condensing lens must be used between the source of light 

 and the paraboloidal condenser, so that its posterior surface is 

 completely and evenly filled with light. Messrs. Zeiss recom- 

 mend a large spherical glass flask of some seven or eight inches 

 diameter, which is filled with water and is used as a collecting- 

 lens. The advantage of this is that it absorbs any heat -rays 

 that might otherwise be a cause of disturbance ; and at the 

 same time a coloured solution may be introduced so that the 

 condensing-lens becomes a light-filter as. well. A one-sixth- 



