MICROSCOPIC IMAGES WITH HIGH POWERS. 259 



oil T ' ¥ 1ST. A. 143. The so-called transverse stria? on the A. 

 Pellucida would remain not only in view, but sharp, under an 

 alteration of focus sufficient to obliterate the bar. 



Some may think that these minute points are unworthy of 

 attention, but they are important, as they are the means by 

 which correct adjustment and focus may be found out. When 

 you have objects such as these in view you cannot be focussing 

 and adjusting on a spectral image. Knowing this, I have been 

 able to see that the white dots, hemispherules, beads, or pearls 

 are not the images of the perforations at all, but are caused by 

 the refractions of the cross pieces of silex between four adjacent 

 perforations. 



These fine details can only be seen by a direct axial cone of 

 large angle. They are completely obliterated under oblique 

 light. I will now, if you will allow me, sum up the lessons 

 taught by this resolution. They are five in number. 



1. There are no such things as markings on the Diatomacece. 

 The so-called markings on the Diatomacece are the structure of 

 the Diatomacea?. One might, with equal propriety, call ribs 

 markings on a skeleton. 



2. The complete destruction of the hemispherule, bead, and 

 pearl theory. 



3. The contradiction of the statement " that you cannot know 

 anything about the structure of the Diatomacece, because all the 

 diffraction spectra are not taken up. 



4. The great superiority of illumination by an axial cone to 

 that by an oblique pencil. 



5. The solution it affords to the questions — What is focus ? 

 What is adjustment ? 



