INDICATOR FOR MICROSCOPES. 61 



hand vertical coinciding with the left-hand vertical of the 

 stage for all numbers from to 50 of the horizontal series ; 

 while from 60 to 110 of the same series the slide must be set 

 so that its right-hand vertical coincides with the right-hand 

 vertical of the stage ; in each case the horizontal lines of the 

 stage and slide being adjusted to coincide. By observing this 

 rule the necessity of perfect accuracy in the position of the 

 guide-lines upon the slides is done away with. 



There are some objections, but not insuperable ones, to the 

 moveable stage Indicator as above described. In the first 

 place, the stage as usually made has its motion too limited to 

 correspond to the whole range of the Indicator ; and secondly, 

 the guide-lines ruled upon the stage for one object-glass may 

 not answer for other powers on account of slight inaccuracies 

 of mounting. 



The stages can doubtless be constructed to give as wide a 

 range for motion as required, which will do away with the 

 first-mentioned objection. The second may be removed by 

 placing an Indicator upon the upper plate of the stage when 

 the latter stands at f~', and adjusting it so that when well 

 centred for the jwivcr employed the line C F shall be parallel 

 to the front edge of the stage. The slide being then placed 

 upon the Indicator, with its guide-lines at ^a' or f^'? the re- 

 maining motions may be made with the screws in the usual 

 manner, and the numbers may be read off from the stage-scales 

 instead of the Indicator. 



The above-mentioned modifications are excellent for in- 

 dividual convenience ; but for the general purposes of science, 

 the compai'able, transferable, reproducible Indicator, in its 

 simplest form, must be preserved ; and it is only in that form 

 that it deserves the name, suggested by a friend, of the Uni- 

 versal Indicator. 



As a proof of the utility and accuracy of the Indicator, and 

 of its convenience as a means of scientific exchange, I may 

 state that numerous mounted slides of minute recent and 

 fossil diatoms have been exchanged through the Post Office 

 by Judge A. S. Johnson of Albany, and myself, and that 

 each has found by the ordinary as well as modified forms 

 of the Indicator all the shells, however minute, fragmentary or 

 previously unknown, which the other had recorded. Some 

 of these objects were less than 1-1 000th of an inch in dia- 

 meter, and yet they were found without difficulty by means of 

 the Indicator. 



To determine whether different impressions of the Indicator 

 when made on the same kind of paper were comparable, a set 

 of objects was registered successively by seven different im- 



