762 . Transactions of the American Institute. 



Testing the Power of Mickoscopes with Nobert's Test Plate. 

 The lines ruled on glass to a known scale by F, A. Nobert of Barth, 

 Prussia, for the pui'pose of testing the resolving power of the micro- 

 scope, are of snch exceeding fineness in the most minnte divisions of 

 space, that the maker has asserted it was impossible that they could 

 ever be seen, and until recently all trials have seemed to confirm the 

 assertion. The ls"obert test is a collection of parallel lines, ruled on 

 glass and arranged in groups or bands of gradually increasing fine- 

 ness. The first group generally contains lines one-thousandth of a 

 Paris line apart — a Paris line being .088815 of an English inch. The 

 number of groups are not the same on all plates, and are not all 

 drawn to the same scale. Thirt}^ groups or bands are sometimes 

 placed on the same plate, and occupy altogether less than one-fiftieth 

 of an inch. Mr. Charles Stodder, in oonnection with Mr. R, 0. 

 Greenleaf, of Boston, recenth^ attempted to resolve the Imes on 'a 

 Nobert test, containing nineteen bands, the first being ruled to one- 

 thousandth of a Paris line, and each band increasing by 500, so that 

 tlie nineteenth is one-ten-thousandth of a Paris line. With a Tolles 

 one-sixtli innnersion objective, angular aperture 170 degrees B., eye- 

 piece power 550, they both saw the nineteenth band satisfiictorily, 

 and were probably the first to see 112,000 lines to the inch ; thus 

 asserting the visibility of such lines, contrary to tlie theory of some 

 physicists. Dr. J. J. Woodward of Washington lately resolved the 

 seventeenth band on a plate of nineteen bands with a Powell and 

 Leland's one-twenty-fifth ar.d one-sixteenth objectives, a Hartnack 

 innnersion, No. 11, and a AVales one-eightli, with amplifier. U]> to 

 the time of this observation Dr. Woodward had resolved finer lines 

 than any other observer had seen. More recently, however, Dr. Bar- 

 nard, President of Columbia College, has experimented with the 

 objectives of Spencer, Tolles, Wales, Nachet, and Hartnack. Remark- 

 able results M'ere obtained with a Wales one-fourth, but it does not 

 appear that he used any stronger power made by this rising optician. 

 Dr. Barnard has satisfactorily seen the lines of the nineteenth band 

 with a JSpencer one-twelfth and a Tolles one-sixth, hotJi dry ohjcctives. 

 This achievement is fairly beyond any point yet reached in Europe 

 or America. These experiments with low powers prove that in suc- 

 cessfully resolviiig fine lines, less de])ends on the magnifying poM-er 

 of the ol)jective tlian on that defining power which results from the 

 skill of the maker. It is certainly gratitying to know tliat the makers 

 of the best microscopes, iSpeucer, ToUe.s, and Wales, reside in this 



