1881.] 



MICROSCOPICAL JOURIs'AL. 



89 



after which they issue from the back 

 surface of the posterior lens converg- 

 ing to form an image at the conjugate 

 focus, where they cross and again 

 diverge. As previously observed, it 

 is customary to say that the objective 

 forms a real image which is magni- 

 fied by the ocular, but when we 

 regard this as the entire process we 

 simply assume that the only result of 

 combining ocular and objective is an 

 increase of magnifying power. The 

 true statement is, that the function 

 of the objective is to focus the 

 function of the eye-piece is to 

 magnify. The superiority of the 

 compound microscope over the 

 simple lens is primarily due to the 

 division of labor which is thus indi- 

 cated. The two functions are so 

 entirely distinct from each other, 

 that the inherent imperfections of 

 one cannot be corrected by imperfec- 

 tions purposely introduced in the 

 other. As already observed, in one 

 plane within the objective, just be- 

 fore the rays undergo the final 

 refraction at the back surface, the 

 rays are proceeding in a parallel 

 direction. The part of the objective 

 engaged in producing this result, acts 

 therefore, like a simple lens forming 

 an image at an infinite distance. 

 These parallel rays could be collected 

 by a suitable telescope. We may 

 consider the focussing function of 

 the objective as complete when this 

 infinitely distant image is formed. 

 The posterior part of the objective 

 may then be regarded as the object- 

 glass of a telescope and the ocular 

 as the eye-piece of the telescope. 

 We would then have a clear concep- 

 tion of the action of the microscope, 

 the objective first forming an infinitely 

 distant image, which is then magni- 

 fied by a telescope consisting of the 

 posterior portion of the back lens of 

 the objective and the ocular. By 

 placing a suitable lens in front of an 

 ordinary telescope, the telescope 

 becomes a microscope. 



When we recognize the specific 

 focussing function of the objective, 



we will at once remark the uselessness 

 of any accessory for correcting its 

 aberrations. Dr. Royston Pigott's 

 " aplanatic searcher," an instrument 

 devised to correct residual aberra- 

 tions, which aroused no little interest 

 in England at one time, failed to 

 accomplish the purpose. The ampli- 

 fier, about which much has been 

 written, only serves to increase the 

 magnification ; it does not add to the 

 optical capacity of the microscope, 

 for it can be clearly shown that for 

 every objective there is a certain 

 amplification that can easily be at- 

 tained by lengthening the body-tube, 

 or by deeper eye-pieces, which will 

 render visible to a normal eye, every 

 detail which the particular objective 

 can resolve ; and no further increase 

 of magnification can do more than to 

 make the same details larger. The 

 explanation of this fact will be given 

 in another place. Returning now to 

 the diatom with which we started, if 

 we use a suitable objective we will 

 see not only the aperture-image, and 

 the dioptric object-image already 

 mentioned, but in addition there will 

 be seen a series of close parallel lines 

 called striae, and these can be made 

 still more distinct by proper manage- 

 ment of the light, or even resolved 

 into series of minute dots. Under 

 these circumstances, if we examine 

 the aperture-image we find no trace 

 of the lined or dotted structure, 

 although the outlines are clear and 

 sharp as before. However, in certain 

 parts of the field, the precise position 

 depending upon the direction of the 

 illumination, one or more spectra are 

 visible, the blue color being nearer 

 the axis than the red. A thorough 

 study of the spectra, and of the aper- 

 ture-image, proves that the decom- 

 position of the light is caused by the 

 diffractive action of the minute mark- 

 ings of the object. The diffraction- 

 spectra which are seen in the 

 aperture-image are the only indica- 

 tions, th^re present, of minute mark- 

 ings on the object. The researches 

 of Professor Abbe have shown that 



