362 OBJECTIVES, EYE-PIECES, THE APERTOMETEK 



adopted on the Continent for low-priced objectives, which led to a 

 reduction of the cost of English objectives of the same construction. 



Manifestly, the single front lessened the risk of technical errors, 

 but we have never been able yet to find a single front objective of 

 the old achromatic dry construction which has shown any superiority 

 over a similar one possessing a triple front. 



The single front employed with two combinations at the back 

 was the form in which the celebrated water-immersion objectives 

 of Powell and Lealand were made. It was by one of these that the 

 striae on Amphipleura pellucida were first resolved. Indeed, what is 

 known as the water-immersion system of objectives, devised by 

 Professor Amici, was the next advance upon the old form ; it should, 

 however, be remembered that as early as 1813 achromatic water - 

 immersion lenses had been suggested by Sir D. Brewster, but it was 

 an advance the optical principles of which were certainly not at the 

 time understood. 



In Paris, Prazmowski and Hartnack brought these objectives to 

 great perfection, and were enabled to take the premier place against 

 all competitors at the Exhibition of 1867. The next year, however, 

 Powell and Lealand adopted the system, and in turn they distanced 

 the Paris opticians and produced some of the finest objectives ever 

 made. Their ' New Formula ' water-immersions were made after 

 the fine model of Tolles referred to below, and had a duplex front, 

 a double middle, and a triple back. In 1877, w r hen the water- 

 immersion system touched its highest point, apertures as great as 

 1'23 were reached; and in America, Spencer, Tolles, and Wales 

 produced some extremely fine lenses of large aperture. 



During the year 1869 Wenham experimented with and sug- 

 gested J the employment of a duplex front ; that is to say, a front 

 combination made up of two uncorrected lenses in contradistinction 

 to an achromatised pair. An illustration of the plan suggested is 

 given in fig. 317, which hardly appears to us as a practicable form, 

 and which certainly was never brought to perfection or put into 

 practice. 



But in the month of August, 1873, Tolles actually made, on 

 w r holly independent lines, a duplex front formula for a !- glycerine 

 immersion of 110 balsam angle, which passed into the possession of 

 the Army Medical Museum at Washington. There can be little 

 :doubt but this objective would have produced a much deeper im- 

 pression but for the fact that it was in advance of its immediate 

 time. 



Tolles, as we have hinted above, used the duplex front in the 

 construction of some of his immersion objectives, and was followed 

 in this by the best English makers, and, in the case of a celebrated 

 ^-inch purchased by Mr. Crisp, Tolles was able to reach a balsam 

 angle of 96. 



At the time that the water-immersion lenses were being con- 

 structed by rival opticians with increasing perfection, the great 

 theory of Professor Abbe concerning microscopic vision, the impor- 

 tance of diffraction spectra, and the relation of aperture to power 

 1 Monthly Micro. Jonrn. Vol. I. p. 172. 



