ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 323 



what of the practical convenience of that in use for objectives, which 

 shall imply a reasonable suggestion not claiming to be a precise 

 statement of the power employed. Though not free from criticism, 

 our nomenclature of objectives has been used, and continues to be 

 used, with some satisfaction, and it is conceded to be the best plan 

 yet brought into actual service. It has been already applied to 

 oculars by several makers and by many users, and it is simple and so 

 in accord with present usage and habits as to seem scarcely an in- 

 novation, and to be capable of adoption without conscious effort or 

 sacrifice. The only other plan seriously proposed, naming by 

 numbers representing the actual magnifying powers, has long ago 

 been tried and abandoned by most competent authorities. It involves 

 such extreme changes of ideas and habits in thinking and speaking of 

 oculars and objectives, that there is great doubt as to the practicability 

 of securing its early adoption. Nor is it certain to be advantageous 

 if adopted. It lacks the simplicity which in the other case classes 

 the powers in a limited number of familiar groups, and introduces 

 an indefinite number of names marked by larger figures, clumsy 

 to use and difficult to remember, and not so easily suggestive of their 

 practical significance. It also involves the claim for a precision 

 which it does not possess, since the complications of collar adjustment 

 and tube-length, which only somewhat impair the value of the approxi- 

 mate method, seem as yet wholly incompatible with a more precise 

 system. Furthermore, the nomenclature by inches would be so easy 

 a movement in the direction of sensible method, that it might prove 

 to be a step toward, rather than from, any further improvement that 

 might prove desirable. 



It is not claimed, and has never been supposed, that the proposed 

 method of measurement would give results exactly corresponding 

 with the optical values of the oculars, as computed by the Cross 

 formula or any other. Nor does it seem certain that persons not 

 opticians can locate the focal planes of their objectives and oculars, 

 with such ease and precision as to secure in each case a tube-length, 

 that would work the various systems exactly at their theoretical 

 power, or that such varying tube-lengths if secured would be within 

 the limits of a convenient working length. 



For these reasons it was suggested to establish a conventional 

 nomenclature, representing as nearly as possible the working value 

 in the Microscope as actually employed, without regard to their value 

 under other circumstances. By this plan a 1/2 in. system would be 

 one which, as used in the Microscope, would double the power when 

 substituted for a 1 in., and a 2 in. would be one which would give 

 one-half the power of a 1 in. This is deemed to be an experiment, 

 for the purpose of obtaining at least temporarily approximate results 

 that would be an improvement on the present practice, in the course 

 of which it is assumed that difficulties will be encountered and 

 intelligent work become necessary, especially in applying it to 

 oculars of exceptional construction. The estimate of powers on the 

 ten diameters' rule would evidently cease to be fairly approximate in 

 relation to objectives of very low power, unless a similar nomenclature 

 should be applied to the objectives also. If the proposed method of 



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