206 NOTES AND MEMORANDA. [Tourna\ 'rprMro.' 



" Messrs. Powell and Lealaud, Smith and Beck, and Eoss, and 

 others of England ; and Nachet, and Hartnack, and others of France, 

 besides many very rejiutable makers in Continental Europe, have 

 honom-ably vied with each other in advancing improvements upon the 

 higher order of objectives. But while such eminent practical skill has 

 endeavoured to meet the wants of scientific men abroad, in pushing 

 their research beyond that of their predecessors, there has been no less 

 demand by restive intellects in this country for the very best instru- 

 ments, and no less intelligence and skill in successful efforts to meet 

 such demand. Among those practical photonomers who have been the 

 most successful, whether at home or abroad, in the scientific construc- 

 tion of the highest order of optical instruments, the samples on exhibi- 

 tion from the Boston Optical Works well warrant the assertion that 

 their superintendent has advanced to the foremost rank. 



" In microscopic objectives, it is obvious that the extent of amjjli- 

 fication, and the character of the light as well as the kind and degree 

 of illumination, are more or less common to all makers. Other things 

 being equal, the relative as well as the absolute merits of objectives 

 are commensurate with their degree of distinctness of delineation or 

 definition. This quality must not merely cover the outline of in- 

 finitesimal objects, so to speak, but extend to the minute details of 

 their structui-e and colom-ing. The higher powers of course more 

 largely magnify any and all defects or imperfections due to their own 

 imperfect construction ; hence the true merits of objectives are in 

 favour of those of the lower powers, which under like circumstances 

 give equal distinctness in definition or resolution. 



" The obstacles to perfection in this direction were various, nu- 

 merous, and enormous, so much so that those savane familiar with them 

 were generally the most decided in pronouncing them all but insur- 

 mountable. But for the high degree of perfection in our present make 

 of instruments, as well as our hopes for still better ones, we are largely 

 indebted to practical opticians, whose perseverance in their study of 

 the laws of light and the principles which prevail affecting the same ; 

 in the material of which the lenses are made, as well as mathematical 

 precision in giving them form, relationship, and combination, was 

 coupled with long series of laborious, but patient trials, studded with 

 discom-aging failures, while on their tedious way to eventual success. 



" The judges took as much pains as circixmstances permitted to 

 compare the workings of the instruments on exhibition with others 

 of the most reputable makers in this country and Europe. The objec- 

 tive of the highest power was the one-tenth immersion, whose angle of 

 aperture is ITS''. Under the observation of the judges, and others 

 whose assistance was invited, this objective [defined test-objects better 

 than any objective of the same power, and as well as many others of 

 higher powers, from other makers, which were at their command. The 

 usual tests were resorted to, such as the Pleurosigma angulatum, Siiri- 

 rella gemma, &c., &c,, among the Diatomaccfe, and Nobert's test-plate 

 from artificial sources. The latter is par excellence the test of the qua- 

 lity of objectives. It consists of straight lines uniformly ruled on 

 glass, and is not subject to the variations which prevail in different 



