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CHAPTER V 



OBJECTIVES, EYE-PIECES, THE APERTOMETEB 



IT is manifest that everything in the form and construction as well 

 AS in the nature of the optical and mechanical accessories of the 

 microscope exists for, and to make more efficient, the special work 

 of the objective, or image-forming lens combination, which constitutes 

 the basis of the optical properties of this instrument. 



The development of the modern objective, as we have already 

 seen, has been very gradual ; but there are definite epochs of very 

 marked and important improvement. Our aim in the study of 

 objectives is practical, not antiquarian, and we may avoid elaborate 

 researches on the subject of non-achroniatic lenses and reflect//';/ 

 specula, which have been sufficiently indicated in the third chapter 

 of this volume. We may also pass over the earlier attempts at 

 achromatism ; the true history of the modern objective begins from the, 

 time that its achromatism had been finally worked out. 



The first movement of a definite character towards this object 

 was made, it has been recently shown, 1 so early as 1808 to 1811 by 

 Bernardino Marzoli, who was Curator of the Physical Laboratory of 

 the Lyceum of Brescia. Mr. Mayall discovered a reference to this 

 effort to make achromatic lenses, and, through the courtesy of the 

 President of the Athemeum of Brescia, discovered that Marzoli 

 was an amateur optician, that he had taken deep interest in the 

 application of achromatism to the microscope, and that a paper of his 

 on the subject had been published in the ' Commentarj ' for the year 

 1808, and that he had exhibited his achromatic objectives at Milan 

 in 1811 and obtained the award of a silver medal for their merits 

 under the authority of the Istituto Reale delle Scienze of that city. 

 One of these objectives was found to have been ' religiously pre- 

 served,' and was generously presented in 1890 by Messrs. Tranini 

 Brothers to the Royal Microscopical Society of London. With it 

 was forwarded the ' Processo Yerbale,' or official record of the awards, 

 notifying Marzoli's exhibits and the award of a silver medal, and 

 the actual diploma, dated August '20, 1811, signed by the Italian 

 Minister of the Interior. 



Marzoli's objective was a cemented combination, having the plane 

 side of the flint presented to the object ; and if this was a part of 

 the intended construction, of which there appeai-s small room for 

 doubt, Marzoli preceded Chevalier in this, as we shall subsequently 

 see, very practical improvement. 



1 Juiinu Roy. Mii: Soc. 1890, p. 4-20. 



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