SMITH AND BECKS FIRST-CLASS MICEOSCOPE. SI 



the planing-machine that ploughs both grooves), the ' secondary ■ 

 body always has its axis so perfectly continuous with that of 

 its principal, that no special adjustment is needed to 'centre' 

 the greater part of the illuminating apparatus. The ' secondary 

 body' or 'cylindrical fitting' is so constructed as to carry the 

 Achromatic Condenser at its upper end, the Polarizing prism at 

 its lower, and the Selenite plates between the two (§ 89) ; it has 

 not, however, any rotatory movement of its own ; but its fittings 

 may be turned in the tube which carries them . The Mirror is 

 hung in the usual way between two centres ; but the semicircle 

 that carries these, instead of being at once pivoted to the tube 

 which slides upon the cylindrical stem, is attached to an inter- 

 mediate arm ; and by means of this it may be placed in such a 

 position as to reflect light very obliquely upon the object. — In 

 regard to weight and complexity, this instrument holds a position 

 intermediate between the Large Microscopes of Ross and of Powell 

 and Lealand, and the Smaller Microscope of the latter makers. 

 Though the mode in which the body is supported has the dis- 

 advantage of separating the focal adjustments from each other and 

 from the stage-motions more widely than is the case in the three 

 preceding instruments, yet the difference is scarcely perceptible in 

 practice. The milled-heads acting on the former are both of them 

 in positions in which they are easily reached by the left hand, 

 when the elbow is resting on the table ; whilst the right hand finds 

 the milled-heads of the traversing stage and of the secondary body 

 in close proximity to each other. — The impei'fection of the means 

 of giving rotation to the Object, and the want of rotatory move- 

 ment in the Sub-stage, constitute points of inferiority to Ross's 

 and to Powell and Lealand' s Large Microscopes : but to those 

 whose objects of pursuit are not of the kind for which these move- 

 ments are specially required, the greater simplicity and less cost 

 of Messrs. Smith and Beck's pattern may afford, an adequate 

 compensation for the deficiency. 



Avoiding invidious comparisons, it may be safely said that 

 whoever desires to possess a First-clctss Microscope, cannot do better 

 than select one of the four instruments last described ; the excel- 

 lence of the Optical performance of the lenses supplied by their 

 respective Makers being — except in the ' case of the very highest 

 powers* — so nearly on a par, that the choice may be decided chiefly 



* The l-12th inch Objective of Mr. Ross bears the reputation of being 

 the most perfect combination yet constructed of that power, at least in 

 this country. Whether it is equalled or surpassed by M. Hartnach*s 

 most recent Immersion-lenses (§ 14) of the like power, has not yet the 

 Author believes) been determined by exact comparison under the same 

 circumstances. — A l-20th inch Objective has been recently constructed by 

 Messrs. Smith and Beck, for the examination of objects that need a high 

 magnifying power, but do not require that extreme of Angular Aperture 

 which involves the employment of the very thinnest covering-glass and 



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