126 Modern Microscopy 



enables us to see with such an objective the markings 

 correctly so far as concerns their number and positions, but 

 any further detail is not correctly presented. Immersion 

 objectives can transmit nearly the whole of the six nearest 

 lateral beams, which are those that would produce spectra 

 of the first order. We now see some detail : the dots 

 appear hexagonal, and are separated from one another by 

 walls which are thin, and which look like a honeycomb. 

 This is the first and the only step we can take towards 

 learning what the actual detail upon this diatom is, since 

 no objective is competent to supply to the image the 

 second or subsequent diffraction beams ; inasmuch as no 

 immersion fluid can shorten the waves of visible light so 

 much as would enable the object to emit and the lens to 

 receive these further diffraction beams. 



The unequal distribution of colour between the several 

 beams is strikingly exhibited by the diatom, known as 

 Actinocyclus Ralfsii. The phenomenon may be conveniently 

 examined through a half-inch apochromatic, over which is 

 mounted an iris diaphragm — an adjunct which is useful for 

 many purposes. Select a frustule which is blue when this 

 upper diaphragm is partly closed. Eemove the eyepiece, 

 close the lower, and open the upper diaphragm. Then, on 

 looking down the tube, it will be seen that most of the red 

 is located in a ring of first lateral beams, with, of course, 

 an equal defect of red in the central beam. Hence the blue 

 colour seen when the image is formed by the central beam 

 only. Now place a small central stop (which may be cut 

 out of card) over the back lens of the objective, open the 

 upper, and partially close the lower diaphragm. The image 

 is then formed by the ring of lateral beams only, and will 

 be found to be preponderatingly red. 



