56 Modern Microscopy 



referred to under the generic term of semi-apocliromatics. 

 Many of these lenses are made in such perfection as to be 

 superior even in some features to the real apochrornatics. 

 Some of the lenses in Watson and Sons' new series of 

 * Holoscopic ' objectives, which require to be used with 

 over-corrected eyepieces of the compensating type, are 

 especially free from spherical aberration. Messrs. Swift 

 and Son, of London, in their series of pan-aplanatic objec- 

 tives, produce beautiful results ; also C. Keichert, of Vienna, 

 and Leitz, of Wetzlar, make objectives that are worthy of 

 special consideration. Beyond these there are most excel- 

 lent series of lenses made by all the microscope manu- 

 facturers which meet the requirements of the ordinary 

 amateur in a most efficient manner in fact, the general 

 quality of such objectives is superior to that which obtained 

 in the so-called best lenses of a few years ago. 



ACHROMATIC versus APOCHBOMATIC OBJECTIVES. In view 

 of the foregoing facts, it will be well to consider which 

 series of objectives should be selected for specific work. 

 It has to be remembered that apochromatic objectives are 

 very expensive, and, generally speaking, are beyond the 

 reach of the ordinary amateur, who usually takes up 

 microscopy without special scientific aims, and excepting to 

 a trained critical eye they would not be found to possess the 

 extraordinary merit that is claimed. 



The question naturally occurs, Is it worth while to incur 

 the great cost which is involved in the purchase of apochro- 

 matic objectives ? No decided opinion can be given without 

 a full knowledge of the scope of the work which is to be 

 undertaken, and as from the nature of things at its incep- 

 tion it is impossible to tell the extent to which research 

 may be carried, the difficulty of giving advice is in- 

 creased. Generally speaking, it may be stated definitely 

 that for the ordinary work of the amateur, the so-called 

 students' series of lenses will be found to give all the 

 pleasure and satisfaction that are to be derived from the 

 examination of Nature's small things, without attempt- 



