The Microscope. 11 



the errors of the screw of the ruling engine, and put the slide in the 

 hands of a well known microscopist who has had much experience 

 in micrometiy, with the request that he measure the spaces and 

 transmit the results to me under cover of a sealed envelope, and 

 then hand the plate to some other competent observer, who should 

 do the same, and so on. No one of the observers knew the results 

 arrived at by the others till the work was entirely completed, and 

 the tabulated results read at a meeting of the State Microscopical 

 Society held in Chicago. The results are quite striking. As will 

 be noticed every observer used a Rogers' stage micrometer, the 

 extreme accm'acy of which may be seen by consulting a prior number 

 of this Journal, Oct., 1885, wherein is published an investigation of 

 ten of the y^j- mm. spaces of such a micrometer, ruled on speculum 

 metal. I suppose that this particular micrometer is no more correct 

 in its ultimate subdivisions than others by this maker used by 

 observers generally, so that some other explanation must be sought 

 for the discrepancy in these measurements (amounting in many 

 instances to more than 1 mikron,) than errors in the standards used. 

 I have ruled another plate with spaces varying from 5. mikron to 10. 

 mikron, to be measured with as high a power as the obseiwer caa 

 command, with a view of determining whether a similar discrepancy 

 will be found in the use of high powers. If a similar discrepancy 

 shall be found upon this trial, the resiilt will tend to shake one's 

 faith in the assertions of some of our so-called experts who pretend 

 to be able to identify a person by the measurement of his blood 

 corpuscles. 



As the investigation is not completed, I simply publish the 

 results so far obtained without further comment than to state that 

 these measurements were not made by novices, but by expert 

 manipulators, most of whom have had long experience in the use of 

 the microscope and micrometer. 



The table needs no further explanation than that the measure- 

 ments are to the nearest hundred- thousandth of an inch. 



The English, rather than the metric system, was chosen for the 

 reason that some of the observers had no metric scale, and it was 

 thought advisable that each series of measurements should be on 

 the same scale as well as entirely independent of the others. Under 

 each measurement will be found the correction " + " or "— " neces- 

 sary to make the measurements equal to the mean. 



I still have the slide in my possession, and if any microscopist 

 would like to measure it, I will mail it to him. 



