Notes on Micro-pJwtograjjhy. By Echvard J. Gayer. 261 



than water are used, the focal distance hetween the objective and the 

 glass cover will be increased, and the screw-collar must be adjusted 

 accordingly to suit the more refractive medium in which the objec- 

 tive has been immersed. When oil of cassia is used (the substance 

 chiefly used by the writer), which has a refractive index as high as 

 1 '405, the screw-collar should be used so as to close the combina- 

 tion completely ; with this latter substance it will be found quite 

 possible to focus through rather thick covering glass. The view I 

 have obtained, with the help of the oil of cassia, of various objects 

 has been very satisfactory. Both the longitudinal as well as the 

 transverse stride on Frudulia Saxonica may be made out fairly. 

 (PI. CXLII.) The longitudinal striae are at the same distance 

 apart as the transverse. I think, from the help I have obtained 

 in this way, and from various experiments I have made, that the 

 markings on at least most of the diatoms are neither elevations or 

 depressions, but are holes, the line of fracture always passing 

 through these holes in the same way as it does through the per- 

 forations round a postage stamp. Diffraction and interference 

 phenomena are so deceiving and perplexing, and look so real, that 

 even photographic proof, as it is sometimes called, goes for very 

 little ; but I hope on some future occasion to be able to ofler 

 satisfactory proofs of the statements I have here made, and regret 

 much that press of time prevents my doing so now. 



