0)1 Zeiss 4^tli Immersion. By W. J. Hiehie. 191 



by the shifting position of the object when the objective is turned 

 round " (p. 163). This operation, however, as it depends for its 

 success upon the precision of the brasswork and the accuracy of 

 the screw of the adapters, is seldom to be rehed on. But the 

 common practice of turning round the eye-piece in the tube is 

 altogether illusory. 



These considerations have suggested to me that possibly, where 

 a foreign objective is tested on a microscope for which it was not 

 constructed, and gets suspected of imperfect centering, the blame 

 may be more justly apportioned between the maker of the micro- 

 scope and the maker of the adapter. The chances, then, of perfect 

 centering, as the phrase is popularly understood, are something 

 infinitesimally small ; and the generality of French and German 

 opticians do not seem to distress themselves to any great extent 

 about its attainment. Their modus operandi is pretty much as fol- 

 lows : — The back combinations are fixtures, put together and placed 

 as fairly as is compatible with a moderate expenditure of labour. The 

 workman then addresses himself to what he regards as the real 

 business of the day, that is, to " marrying lenses," as he calls it. In 

 other words, he tries on front after front, taken almost at random 

 out of a great number, till he hits upon that one which exactly ac- 

 commodates itself to the previous combinations, these fronts being, 

 as an optician expressed himself to me, "made in bushels; yes, 

 sir ; made in bushels by girls." I could give, if it were desirable 

 to do so, a goodly list of French and German opticians who follow 

 this facile plan of manufacturing " first-class objectives." On the 

 other hand, I could only mention two with any confidence, who, to 

 judge by their workmanship, appear to follow a better advised 

 method, and these two are Zeiss and Seibert. Glasses made on the 

 first plan are easily recognized. Let the intending purchaser only 

 hold them up to the light, and he cannot fail to notice the enormous 

 disproportion between the breadth of the front lens and the breadth 

 of the hindermost lens. There is, of course, a trick in this. As 

 spherical aberration turns upon the fact that the focus of the peri- 

 pheral rays is always shorter * than that of the central rays, which 

 disparity increases, pari passu, with the distance from the axis, f 

 it is quite conceivable that the adaptation of an extremely minute 

 front lens to wide back combinations may shorten the labour of 

 correction very considerably. Such glasses, when used with a low 

 eye-piece, sometimes exhibit very brilliant definition ; but then it is 

 only within a certain limited range of the correction-screw. They 

 will also generally be found, in spite of their good definition, to 



* Thus, if we divide the periphery into a number of zones, the rays < f the outer 

 zone will have a shorter focus than those of the next zone, and the rays of the 

 second zone a shorter focus than those of the third zone. See Nageli, /. c. 



t Nageli and Schwendener, p. 43. 



