36 HOW TO WORK 



have been ruled upon slips of glass by means of a diamond attached 

 to a beautiful instrument, provided with a most delicate arrangement 

 for moving it the required distance from the last line engraved. A 

 second line is then ruled, then a third, and so on. Excellent stage 

 micrometers of this kind have been ruled by the late Mr. Jackson. 

 They can be obtained of all the instrument makers ; but they are 

 made by Messrs. Powell and Lealand. 



61. Test Objects. To such wonderful perfection has this process 

 been carried, that M. Nobert of Griefswald, in Prussia, has engraved 

 lines upon glass so close together that more than 100,000 would go 

 in the space of an English inch. Several bands, each containing 

 many lines equidistant from one another, were engraved upon one 

 slip of glass, but the lines in each different band were separated by 

 gradually diminishing intervals, constituting a series which could be 

 readily submitted to examination one after another. By aid of these 

 the defining power of any object-glass could be estimated. As test objects, 

 they are equal to, and even rival, many natural objects which have 

 hitherto been employed for this purpose. The delicate lines on some 

 of the diatomaceas are separated from each other by the i-5o,oooth 

 of an inch, while the finest lines engraved by M. Nobert are less 

 than the i-ioo,oooth of an inch apart. 



In order to measure the diameter of an object the glass slide 

 upon which the lines have been engraved (i-ioooth or i-iooth of 

 an inch apart according to the magnifying power) may be placed 

 beneath the object upon the stage. This arrangement, however, 

 is only suitable for low powers, since the object and lines cannot 

 be in focus at the same moment, and it is, therefore, impossible 

 to obtain a very correct measurement. 



The podura scale is a most excellent " test object." According 

 to Prof. Bailey of the United States, Grammatophora subtilissima 

 and Hyalodiscus subtilis, are the most delicate tests (" Smithsonian 

 Contributions," vols. II and VII ; also a paper by Mr. Hendry, 

 "Quart. Journ. Mic. Science," vol. I, p. 179, 1861 ; one by Messrs. 

 Sullivant and Wormley, " Silliman's American Journal," Jan. 1861). 



For testing the penetrating power of an object-glass, very fine 

 nerve fibres lying on different planes, as, for example, those distributed 

 to vessels, or very delicate fibres of striated muscle, mounted in glyce- 

 rine, may be employed. It should be borne in mind that the object- 

 glasses with a very high angle, although very valuable for researches 

 upon the diatomaceas, and other delicate objects of extreme 

 tenuity, do not answer so well for investigations upon the structure 

 of animal and vegetable tissues, as glasses of a moderate or very 

 low angle. This question is fully discussed in the remarks on 



