ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



809 



of such a large dioptric beam, are those little edges of spectra in the 

 peripheral zone so sui)remely important ? 



(4) You cannot have an irregular picture from a spectral image. A 

 short time ago I also held this view, and I used to assign the irregu- 

 larities in the microscopic image to a function of the dioptric beam ; but 

 special experiments, made with a view to determine this point, have 

 altered my opinion. In a purely spectral image I have seen irregularities 

 in the microscopic image, such as a missing dot, &c. These ditferences 

 are not clearly seen, and yet they are seen. I do not for one moment say 

 that the dioptric beam has no influence on the image — it has a very great 

 influence ; in fact, a greater influence than perhajjs any spectrum taken 

 by itself; but that is quite another matter altogether to saying that the 

 microscojjic image is a purely dioptric one. 



(5) The reflex from the front^lens. This is the only part of the 

 dioptric theory which has been supported by experiment. It is very well 



Fm. 104. 



Fig. 105. 



Fig. 106. 



Fig. 107. 



Explanation of Figs. 



Fig. 104. — Shows back of objective, with spectra of first order of P. angulatum, 

 the dioptric beam of small angle being stopped out. The diatom will be resolved on 

 a dark ground, and will be fairly bright. 



Fig. 105. — The same, with a dioptric beam of larger angle. The diatom will be 

 resolved and intensely lighted on a dark ground. 



Fig. 106.— The same, with a smaller aperture, so as to admit only the edges of 

 the expanded spectra. The dioptric beam is now present, and the diatom is resolved 

 on a light ground. 



Fig. 107. — The same, aperture of lens reduced, so as to cut out the edges of the 

 spectra ; dioptric beam same as in fig. 106. The diatom is not resolved. 



known that an object, such as a diatom, illuminated by a central axial 

 cone, appears brighter than the field. 



The " dioptricians " explain this fact by saying it is caused by light 

 reflected from the front lens of the objective, and this statement is sup- 

 ported by the experimental examinations of opaque objects mounted in 

 balsam. 



I have very grave doubts as to the opacity of some of these objects 

 which shine so brilliantly ; therefore, let us pass on to one object upon 

 which there can be no doubt — viz. the mercury globule. 



On examination, a mercury globule exhibits a feeble illumination from 

 the reflected light. A great deal depends, however, on the curvature of 

 the front lens, which, of course, difl"ers in lenses of different construc- 

 tions. It was found on trial that a certain dry 1/4 gave brighter 

 illumination than another dry 1/4, also both 1/4's gave more brilliant 

 results than a certain oil-immersion 1/8. It was also found that the 



