ON DIFFRACTION SPECTRA IN THE MICROSCOPE. ^^ 



ray which are consequently coherent with each other (vibrate in 

 equal phase). It is, therefore, immaterial to the effect — apart 

 from the greater or less amount of brightness — what number of 

 direct rays enter. If the diaphragm slit is rightly apportioned and 

 in proper position, it exercises the same effect upon any number 

 of diffracted rays that may be thrown off from the whole mass of 

 incident rays entering the objective. For instance, the diaphragm 

 card No. i (three slits,) when' a direct ray enters through each 

 point of its openings, shuts off the next situate diffracted ray (of the 

 systems of wide lines,) and as regularly lets pass the next situated 

 diffracted ray of the system of narrow lines, provided the three 

 slits be parallel with the lines, (see pi. II., fig. 3 and 4.) 



Experiment IV. 



Focus the microscope upon the groups of lines crossed at a 

 right angle, then affix diaphragm 1 c, and examine the object 

 under three different positions of the slit, when the following 

 changes will be seen. If the slit be horizontal the vertical lines 

 disappear : if the slit be vertical the horizontal lines will disappear : if 

 the slit be inclined at 45° to vertical (either right or left side) all the 

 real lines disappear, and diffraction lines appear everywhere 

 inclined at right angles to the slit, and therefore inclined diagonally 

 to the crossed vertical and horizontal lines, (pi. III., fig. 12.) 



Experiment V. — 



When the group of crossed lines is treated in the same way, the 

 disappearance of all real lines at right angles with the slit, and the 

 appearance of diffraction lines which run diagonally across the 

 rhomboids joining the opposite angles (acute or obtuse according 

 to the position of the slit) is, as in experiment IV., distinctly 

 observed, (pi. III., fig. 13.) In both experiments, also, the new 

 diffraction lines are close together and finer in calculable proportions 

 than the real lines, (see Abbe's essay translation in Part II., Vol i, 

 B. N. S. Proceedings.) 



