1881.] 



MICROSCOPICAL JOURNAL. 



73 



objectives have far excelled any 

 others. It was first practically intro- 

 duced by Carl Zeiss, of Jena, who made 

 lenses according to formulae calcu- 

 lated by Professor E. Abbe, at the 

 suggestion of Mr. John Stephenson, 

 of London. 



The compound microscope con- 

 sists of two parts, an objective and an 

 eye-piece or ocular. The objective 

 collects the light from an object 

 placed at its anterior focus, and forms 

 an image of that object at the conju- 

 gate focus. This image is enlarged 

 by the ocular. The combination of 

 objective and ocular affords an image 

 that is far more perfect than it is 

 possible to obtain with a simple lens, 

 even when the amplification is no 

 greater than a simple lens would 

 readily give. . 



Until within a very few years, it 

 has been assumed by all writers on 

 the microscope, that the magnified 

 image in this instrument is formed 

 according to dioptrical principles, 

 such as govern the course of the 

 light through a telescope ; and even 

 to-day there is no text-book that con- 

 tains any information whatever, 

 about the recent researches which 

 have shown the fallacy of this as- 

 sumption.* " According to the old 

 theory, it was supposed that the 

 image-forming rays which entered 

 an objective underwent a series of 

 refractions and formed an image of 

 the object at the conjugate focus, 

 then continued on to the ocular, 

 which was so constructed that a cer- 

 tain over-correction of the objective 

 would be neutralized, and the image 

 was thus rendered free from errors 

 of color. It was supposed that all 

 structures, no matter how minute or 

 delicate they might be, were imaged 

 in this manner, by what is known as 

 the dioptrical process. 



Since many of the fine markings of 

 diatom-frustules, which are easily re- 

 solved by ordinary lenses, are sepa- 

 rated by spaces much smaller than 



* Nageli und Schwendener's treatise on the 

 microscope should be excepted. 



the wave-length of blue light, it is 

 difficult to understand how such 

 markings could be imaged dioptri- 

 cally. The objective and ocular 

 were supposed to act together as one 

 optical system, and it was assumed 

 that the defects and imperfections of 

 one, could be remedied by modifica- 

 tions of the other. According to this 

 theory, the only purpose of the ocular 

 was to add to the power of the ob- 

 jective ; consequently, the theory 

 afforded no explanation of the ac- 

 knowledged superiority of the com- 

 pound over the simple microscope. 

 As to the aberrations, none but spheri- 

 cal and chromatic aberrations were 

 taken account of in their simple 

 forms. Generally speaking, when 

 the definition in all parts of the field 

 was good, and required no change of 

 focus to make any part distinct, the 

 glass was considered free from spheri- 

 cal aberration or curvature of field. 

 The chromatic aberration was in- 

 dicated by the color of the image, 

 and the appearance of the borders. 

 ( To be continued?) 



EDITORIAL. 



— Evidently The Northern Micro- 

 scopist (London) has been canvassing 

 for advertisements, for the last cover- 

 page of a recent number bears the 

 following card : — 



"Some Hints on Advertising.— 

 American paper-makers have not been 

 slow to avail themselves of the newest 

 and best ideas for improving their indus- 

 try and developing their trade, but they 

 have singularly neglected one funda- 

 mental principal of business, which the 

 keen and pushing merchant finds of great 

 practical value. Very few paper-makers 

 consider the worth of advertising, and 

 many forget that a journal devoted to 

 their interests has claims upon them 

 which they ought to acknowledge. — Paper 

 Trade Journal. 



For American paper-makers read 

 English opticians in the above paragraph," 



" The bearings of this observation 

 lays in the application on it." 



