in common use as Test- Objects. 123 



themselves^ in order to arrogate for their workmanship a degree 

 of excellence it in nowise merited. 



It should be borne in mind that the mere number of lines in 

 any given fractional portion of an inch is not all that is required 

 in a Diatom employed as a test. Much depends on the parti- 

 cular conformation of the Diatomaceous valve — on its thickness, 

 flatness, the angularity of its markings, their direction, the kind 

 of illumination, and so forth. But this only tends still further 

 to diminish the value of such objects as tests, inasmuch as the 

 same form, under different conditions, may or may not be suited 

 to test a given combination. 



It is not my province to discuss how much valuable time is 

 often wasted in the endeavom" to resolve markings on Diatoms 

 or other minute objects, that might be applied to higher and 

 far more useful purposes. It is well known that many ardent 

 observers labour, for days and even weeks together, in order to 

 conquer some difficulty of the kind, — their toil ending in failure 

 simply because they are unaware that the same species of test- 

 Diatom may present itself under such modifications as to baffle 

 aU attempts at resolution in one shape, whilst it may be made 

 to yield readily in another. At present the capabilities of lenses 

 have, too frequently, to be discovered after purchase. They may 

 or they may not come up to the professed standard. The owner 

 may apply an incorrect or an insufficient kind of test ; and, 

 under any circumstances, he is unable positively to assure himself 

 of the real power of the apparatus he is using. 



Certain Diatoms may still be advantageously employed as 

 test-objects, but assuredly not in the manner hitherto in vogue. 

 In order to ensure uniformity, or, what amounts to the same 

 thing, in order to ensure the purchaser of a lens of a stated 

 power actually obtaining what he desires, it becomes essential 

 that each test-slide should itself be compared with some accre- 

 dited and universal standard, before being applied to the decision 

 of the capabilities of any optical combination. 



I shall endeavour presently to show how likely we are to be 

 misled in our estimate of lenses based upon the resolution of 

 some of the ordinai'y tests, by giving a tabular statement of the 

 range of lineation admitted to exist by those who have given the 

 greatest amount of attention to the markings on the various test- 

 Diatoms. In dwelling on this topic 1 have an important pur- 

 pose in view, namely, the introduction of a definite and uniform 

 standard test, for each grade of objective, in lieu of the inde- 

 finite and variable ones that have heretofore been in use. 



M. Nobert's test- glasses at once suggest themselves to our 

 notice as likely to afford the requisite standard. We have here 

 a degree of minute workmanship destined for ever to defy the 



9* 



