ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 2^ 



minuter structures seen through the microscope, which are formed 

 by the addition of diffraction spectra to the ordinary dioptric or 

 direct image. It must further be remembered that the deviation 

 of rays produced by minute structure of objects, and treated in the 

 the preceding article — as also by Professor Abbe in his essays — as 

 "diffraction" (the typical example of which is that produced by 

 gratings of various kinds on beams of light transmitted through 

 them) is likewise produced when light is transmitted through 

 membranes consisting, throughout, of transparent substance, in 

 which finely striated thickenings occur. 



This is important to bear in mind when the microscope is 

 employed for histological investigations, and the minute analysis of 

 all organic substances. Interference phenomena are also produced 

 by striation which arises from greater and lesser degrees of moisture 

 in alternate parts of membranes. Every optically dense streak 

 line or surface projection in such membranes must act as a 

 cylindrical lens, forming a '*" positive" image of the light source which 

 presents itself to the objective as a separate self-luminous line or 

 fissure. Effects of this kind must frequently occur in cell 

 membranes, insect scales, diatoms, &c. 



Other interference phenomena are discussed in the chapter, to 

 which reference has been already made, under various heads, as 

 follows: (2) — Reflections of light from minute particles, globules, 

 fine threads, hairs, &c. (3) — Interference bands surrounding 

 certain objects upon the withdrawal of a luminous pencil from the 

 plane of focus of the microscope, as, e.^., in focussing above or 

 below the object. (4.) — Interference in the focal plane as 

 produced by («,) direct with reflected light ; (b,) dispersed with 

 reflected light. 



Referring to Vol. I., part II. pages 232-245 of our Proceedings 

 for the general statement of principles and facts established by 

 Professor Abbe, we now turn to the practical demonstration of 

 '' diffraction spectra in the microscope,'' and shall briefly describe 

 the method employed, adding a few hints relating to the 

 management of the instrument, illumination, &c. The writer 



