OPTICAL PRINCIPLES OF THE MICROSCOPE. 



Plate IT' 



ECHINUS-SPINE (Original), AND PODURA SCALE (after R. Beck). 



Fig. 1. Transverse section of Spine of Echinometra heteropora. 



2. Markings on Scale of Podura, as seen by transmitted light under a well -corrected l-8th inch 

 Objective. 



3. Partial obliteration of the markings by the insinuation of moisture between the Scale and the 

 Covering-glass. 



4. Appearance of the markings, when the Scale is illuminated from above the oblique light 

 falling at right angles to them. 



5. The same, when the light falls on the Scale in the direction of (he markings. 



