1884.] 



MICROSCOPICAL JOURNAL. 



193 



Place a dry-mounted specimen of 

 Pleurosigma For7nosum under a 

 microscope and examine it with a 

 half-inch objective. When the mark- 

 ings are distinctly seen, remove the 

 ocular and look dow^n the tube of the 

 microscope. There will be seen the 

 diffraction spectra, just above the ob- 

 jective. Then put on the ocular and 

 with a hand-lens examine the light 

 which issues from it in the same way. 

 There the same spectra will be found 

 again. I will not attempt to explain 

 how it is that these spectra are so 

 combined in the image we see to pro- 

 duce the appearance of lines or dots as 

 the case may be. The subject is too 

 intricate to be easily understood ; but 

 the fact must be taken upon the au- 

 thority of Prof. Abbe and others that 

 the power of a microscope to define 

 or resolve minute markings is abso- 

 lutely dependent upon its capability 

 of gathering in, and properly dispos- 

 ing of, the spectra produced by the 

 object. 



We will now go a step further in 

 the consideration of the formation of 

 the spectra. You are all familiar with 

 the fact that diffraction spectra are of 

 different orders — that is to say, we 

 have several secondary spectra pro- 

 duced, one beyond the other, more or 

 less overlapping each other. So it is 

 under the microscope, and the number 

 of secondary spectra will depend upon 

 the fineness of the markings of the 

 object and the length of the vibrations 

 of the light (which will vary in dif- 

 ferent mounting media) . It scarcely 

 need be said that when the object is 

 so finely marked as to produce two 

 spectra the microscope must take in 

 both of these to reveal its finest lines, 

 and if tlie object produces three spec- 

 tra that are essential to show its 

 structure, the microscope must take 

 the third in also Since the rays 

 forming the second and third spectra 

 are difiracted respectively further from 

 their courses than the others, it is ob- 

 vious that the capability of an objective 

 to take them in depends upon its an- 

 gular aperture — the angle of light 



which it will take in under the con- 

 ditions of experiment. 



We now come to an interesting and 

 exceedingly important fact concerning 

 the appearances produced by diffrac- 

 tion spectra. If an objective is inca- 

 pable of taking in the spectra necessary 

 for a true portrayal of the object, it 

 may, nevertheless, be made to give 

 an image which, to all appearances, 

 is true, and would not give any evi- 

 dences of further details to be brought 

 out by other lenses. It may also 

 happen that by proper manipulation 

 an objective capable of showing the 

 finest markings of an object may be 

 made to show the object clearly de- 

 fined with only one-half the true num- 

 ber of markings — the structure ap- 

 pearing to be much coarser than it 

 really is. 



It may be asked how it is possible, 

 in view of such facts as these, to place 

 greatdependence upon the microscopi- 

 cal appearance of objects — how are we 

 to be sure that the resolutions we see 

 of fine markings are true, how do we 

 ever know that the objectives we use 

 are capable of showing true structures, 

 or that our manipulation of them has 

 given us the true and not the false .'' 



It is not my purpose to enter upon 

 a discussion of these questions. It 

 would require too much time and in- 

 volve a more full discussion of the 

 theory of the microscope. Sufficient 

 has been said to place the observer on 

 his guard against errors of this kind. 

 In some cases structure cannot be 

 known from microscopical examina- 

 tion. 



We now come to the consideration 

 of angular aperture. The cone of 

 rays radiating from the focal point of 

 the objective to the periphery of the 

 front lens is the angular aperture — the 

 angle of light it will receive. Sup- 

 pose an object to produce a diffraction 

 spectrum just beyond the range of a 

 certain objective working in air. In- 

 terpose a refractive medium like gly- 

 cerin or water between the object and 

 the front lens. The rays which be- 

 fore were beyond the range of the 



