OBJECTS AND THEIR MOUNTING. 41 



should be remarked, that when the objects are used as opaque, a 

 smaller aperture will do best, namely about two-fifths of its focus. 

 For transparent objects, a larger aperture is absolutely necessary ; and 

 for some tests it should be equal to its focal distance, to show the cross 

 striae between the lines on many of the scales when the power of the 

 instrument or lens is considerable. It is worthy of remark, that the 

 same aperture that with advantage will develop one class of objects 

 will not show another with the same success. 



30. As a conclusion to the subject of test objects, I shall quote the 

 valuable remarks of Mr. Pritchard, in a little work published some 

 time since: "It cannot too strongly be impressed upon an observer, 

 that of two instruments, the one which shows the object with the 

 least power is always the best This, of course, will not suit the lovers 

 of the marvellous, who would rather see a flea appear as big as an 

 elephant, though they lost all its finer markings, than have them pre- 

 served when moderately magnified. Another class of observers, who 

 admire high powers for the microscope, are those whose sight is so 

 defective that they cannot otherwise see a very minute object." 



CHAPTER IV. 



MICROSCOPIC OBJECTS AND THEIR MOUNTING. 



31. Every department of nature is full of objects, from the 

 examination of which the most important discoveries have already- 

 been and still may be expected to be made. But though the zealous 

 observer can never be at a loss for subjects of research, it is desirable 

 to know what has been done by our predecessors, and what trains of 

 enquiry are likely to prove of the most general interest ; for it is more 

 particularly for the use of the general examiner that this little treatise 

 has been written. It may, however, be again necessary to remind 

 the reader, that as the work is intended only as a practical guide to 

 the microscopical student, it is not the intention of the author to enter 



