ON INTERFERENCE IN THE MICROSCOPE. 23 



at right angles, then appear two additional striations in the 

 direction of their diagonals. Lines that lie in the same level will 

 often be seen with different setting of focus, or vice versa. It may 

 even happen that a single system of striae will be reformed in 

 double or triple repetitions. All these phenomena may be 

 theoretically calculated and constructed beforehand. They belong 

 to the well-known chapter of diffraction phenomena produced by 

 means of gratings, concerning which the necessary information is 

 given in every hand-book on physics. From this it follows that 

 the visible indications of structure in a microscopic image are not 

 always or necessarily conformable to the actual nature of the object 

 examined. Of course, therefore, every attempt to discover the 

 minute structure of organised objects, — as, for instance, the 

 frustules of Diatoms — by microscopic investigation alone must be 

 considered insufficient. 



