84: F. CRISP ON THE INFLUENCE OF 



were now brought within it, the lines of the object would at once 

 become visible, and the latter object-glass would have in ordinary 

 parlance greater " resolving power " than the former. 



The advantage, therefore, of an object-glass of large angle over 

 one of smaller angle consists, in the case of objects with minute 

 details, entirely in the fact that spectra, which with the one are thrown 

 outside its field, are with the other brought within it, and thereby 

 details of the object become visible. 



We are consequently at once able to dispose, on the clearest 

 grounds, of the controversy, raised a year or two back, as to the power 

 of small-angled object-glasses to show the finest lines. Those who 

 asserted such a fact were evidently (however unconsciously) drawing 

 upon their imagination. 



One other experiment may be mentioned as extending somewhat 

 the ideas which the preceding ones enable us to form, and will be 

 interesting to Microscopists as bearing upon the question of the 

 structure of P. angulatum. 



If an object is taken which is formed of two sets of lines placed 

 over one another at right angles, we get of course a series of squares, 

 as shown in Fig. Ill, which produces the spectra of Fig. IV. 



It must be remembered (a point which I have not mentioned 

 before, as its application is better seen here) that the spectra are 

 always produced in a direction at right angles to that of the lines, as 

 is seen in Figs. I and II, where the lines are vertical, whilst the 

 spectra are horizontal. In the case of the cross lines we are now 

 considering, the horizontal spectra are due to the vertical lines, and 

 the vertical to the horizontal ones. So a line inclined at an angle 

 of 45° — say, for instance, a diagonal drawn from the top left to 

 the bottom right-hand corner of Fig. Ill, would produce spectra 

 ranged in a line parallel to the other diagonal. 



It will occur to you that these do actually exist in Fig. IV, and 

 that, therefore, if we place at the back of the object-glass a 

 diaphragm like Fig. D, only the spectra along the one diagonal will 

 be admitted. When this is done, the rectangular lines entirely dis- 

 appear, and are replaced by lines parallel to the other diagonal, as 

 shown in Fig. d — a very striking example of the production of 

 spectral lines. If the diaphragm had two slits at right angles, a 

 second set of lines at right angles to the former would have 

 appeared, forming a network smaller than the original ; so that, in 

 lieu of the true object with vertical and horizontal lines, we should 



