62 ON A UNIVERSAL INDICATOR FOR MICROSCOPES. 



pressions made on enamelled cards, some of which were 

 arranged with the ordinary paper centre-piece, and others with 

 the central guide-lines ruled upon glass. The numbers being 

 recorded for the objects when well centred upon one of these 

 Indicators, the slide was then transferred to each of the other 

 Indicators, and each object being brought into the field by its 

 recorded numbers, the position was carefully adjusted so that 

 the object should be well centered, and a record for each copy 

 of the Indicator was thus made. On comparing the different 

 numbers it was found that the coincidence was almost perfect, 

 the difference never exceeding one-fourth of one of the divisions 

 of the Indicator, an amount which might be quadrupled before 

 an object would be thrown out of the field of view of my 

 •J-inch objective. 



The Indicator having been put to so many and such severe 

 tests, I feel no hesitation in recommending it as a means of 

 scientific intercourse among observers, and as a means by 

 which collections of microscopic objects may be registered, 

 arranged, and catalogued ; and an index to the whole so made 

 that any particular specimen may be found at will either by 

 the original observer or any one into whose hands the slides 

 and accompanying register may at any time come. 



The copy of the Indicator which accompanies this paper is 

 not given for use with the microscope, as the kind of paper 

 upon which it is printed is different from that used for the 

 standard Indicator, and therefore in consequence of unequal 

 shrinkage a slight deviation is produced. The Indicator for 

 use with the microscope is printed upon enamelled cards, and 

 the different impressions have been found to agree so closely 

 with each other as well as with the original plate that no 

 appreciable error is pei'ceived. 



I cannot close this paper without expressing my warm thanks 

 to Judge A. S. Johnson, of the New York Court of Appeals, 

 for his cordial sympathy and aid in testing the merits of the 

 Indicator, and for some excellent suggestions as to its best 

 form for general use. I should also express my obligations to 

 the engraver, J. E. Gavit, Esq., of Albany, who has spared 

 no pains in making the steel plate from which the Indicator is 

 printed as accurate as possible. 



