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An Easy and Simple Method of Resolving the Finest-Lined 

 Dry Diatomaceous Tests, mounted on Cover, with special 

 Reference to Amphi pleura pellucida. 



By Adolf Schulze. 

 {Bead April 25th, 1879.) 



The object of this article is to call attention to an excellent method 

 of resolving the finest-lined tests, mounted dry and on cover. This 

 mode of illumination I discovered in the course of my attempts at 

 resolution, but I doubt whether I have been the first to employ it, 

 as it is really so very simple that others can not have failed to 

 discover it too, but as I read and hear continually of the great diffi- 

 culties in " checking ' : Frustulia saxonica, and in u lining " 

 Amphipleura pellucida even with the best objectives I cannot but 

 assume that the method is not generally known, hence my reason for 

 describing it. 



When I succeeded in resolving Amphipleura pellucida in balsam, 

 by means of the Reflex Illuminator, I became aware of the great 

 value of immersion condensers as a means for the utilisation of the 

 full apertures of immersion lenses, but I saw at the same time that 

 substage immersion condensers would only be of value for objects 

 mounted in balsam or for objects mounted dry on the slip, whilst for 

 objects mounted dry on the cover they were practically valueless^ 

 because rays of the requisite obliquity, emanating from the substage 

 immersion condenser, have so great obliquity that they are totally 

 reflected internally from the upper surface of the slip, and so never 

 pass through the layer of air to the object. It was, therefore, clear 

 that immersion condensers for the oblique illumination of objects 

 mounted dry on cover could not be substage, but must be super stage 

 ones, i.e., condensers joined to the upper surface of the cover to which 

 the diatom or other object adheres. The immersion objective itself 

 presented the first claim as a superstage immersion condenser, and 

 the oblique illumination was readily obtained by the interpolation of 



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