158 



THE MICROSCOPE AND ITS REVELATIONS. 



sentation at all of the lined surface; whilst (2) by varying the combinations of the- 

 diffraction-spectra, the lineation shown in the Microscopic image of the ruled, 

 surface may be partially or completely changed. 



Experiment 1. If, in the first place, a diaphragm \vith a single diametric slit 

 (B) be so placed immediately behind the Objective, that the slit is parallel to the. 



FIG. 117. 



direction of the ruled lines thus giving passage to the direct rays forming the 

 dioptric image, but excluding all diffraction-spectra the field seen through the 

 replaced eye-piece shows no lineation ivhatever, the ' grating ' being replaced by a 

 plain silver band. Yet, if the diaphragm be turned a quarter round, so that the 

 slit lies transversely to the lines, and admits the pairs of diffraction-spectra in 



