THE OPTICAL ARRANGEMENT. 263 



are accompanied by the similar disadvantage of a short focal 

 distance ; nevertheless, they must always be regarded among the 

 best and least costly magnifying-glasses. If the stronger curvature 

 is turned to the object a greater object- distance is obtained, but 

 the aberrations are then notably augmented, and the images 

 consequently less distinct. 



In recent times, since about the year 1830, systems composed 

 of two or three plano-convex lenses (the so-called doublets and 

 triplets) have come more into favour. It is true that earlier 

 optical physicists, especially Euler and John Herschel, had already 

 occupied themselves in developing such combinations of lenses, 

 but with the impossibility of satisfying the results of calculation 

 in the grinding of the lenses, the theoretically accurate proposals 

 were of little practical importance. Wollaston was the first to 

 discover a means of increasing the sharpness of the image rather 

 by the relative distance of the lenses, deter- 

 mined by experiment, than by their particular 

 form. His doublets consist of two plano-convex 

 lenses with the plane surfaces turned down- 



wards, their focal lengths in the ratio of about FlG * 146 * 



3:1, and placed one above the other in such a manner that the 

 distance of the plane surfaces is about T5 of the shorter focal 

 length (Fig. 146). The success of these doublets, and the atten- 

 tion which they aroused, soon led to further improvements. 



Pritchard found the most favourable distance of the two lenses 

 to be equal to the difference of their focal lengths, but that these 

 focal lengths might vary within certain limits (e.g., between the 

 ratios 1 : 3 and 1:6). He constructed doublets, which Mohl charac- 

 terizes as remarkable for the clearness and sharpness of the image, 

 up to an amplification of about 200 linear, and of which the 

 low-power ones are especially well adapted for dissections, &c. 

 The highest powers, which magnify somewhat more than 300 

 linear, exhibit distinctly the sixth band of Nobert's (30-band) 

 test-plate. 



Chevalier combined two plano-convex lenses of equal focal 

 length but unequal size (Fig. 147), so that the 

 larger was nearest to the object. Between 

 the two was placed a diaphragm d with the 

 aperture o. A considerable intensity of light 

 was thus obtained, and at the same time a FlG> 



