The Microscope. 77 



It may, perhaps, be objected that the foregoing deductions can 

 not properly be made for the reason that the conditions of mi- 

 croscopic vision are entirely different from those of ordinar}' 

 vision. The writer is fully sensible of the force of these ob- 

 jections ; nevertheless the mechanical difficulties in the way of 

 manufacturing a similar test object of a definite size so small as 

 those above are so great as to be insuperable, and the writer 

 knows of no other way of arriving at the conclusions here drawn 

 unless it be actually to measure the smallest point visible under 

 the microscope with different powers. It was the intention of 

 the writer to include in this paper the results of such measure- 

 ments ; but the want of time has prevented the accomplishment 

 of this work, the results of which must be reserved for a future 

 communication. There are, however, so many sources of error 

 in the measurments of such minute objects that, as it seems to 

 the writer, the errors in the results would be quite as great, if 

 not greater, than in the results here recorded. Whereas in the 

 latter the object observed is so large as to present no mechanical 

 difficulties in its manufacture. No reference is made in the fore- 

 going observations to the resolution of finely ruled lines but 

 only to the objects described. It is quite likely that a different 

 combination of colors, would be followed by different results 

 but this must be left for future observation. 



REFRACTIVE INDEX AND MOUNTING MEDIA. 



PROFESSOR A. B. AUBERT. 



JUDGING from the communications occasionally seen in mic- 

 roscopical journals on the subject of increasing the refrac- 

 tive index of mounting media, I am inclined to think that a 

 misconception exists in the minds of some as to the methods by 

 which this end can be attained. Thus, for instance, one writer 

 indicates the possibility of treating some of the oils and resinous 

 media with lead chromate, expecting thereby, I suppose, to in- 

 crease their refractive indices. Such, however, would only be 

 the case if the lead chromate were soluble in these media. 



The easiest method of obtaining highly refractive media, is to 

 dissolve a highly refractive substance in a highly refractive sol- 

 vent, which will not act chemically, but produce a solution of 

 the requisite consistency. 



