IMAGES OF WIKE-GAUZE AS TEST-OBJECTS. 129 



sides half the thickness of a wire) is exactly one-tenth, that is 

 75 mic. 1 



For fixing the wire-gauze a holder, consisting of a horizontal arm 

 provided with a ring, which is movable on a vertical arm, is the 

 most convenient ; or it may be attached by means of wax to the 

 lower end of a cylinder-diaphragm, and the distance regulated by 

 sliding the carrier in the socket. This method of testing presents 

 no difficulties ; it is only necessary to carefully select air-bubbles 

 that yield clear and sharp images. 



We give on pp. 130-1 a table of the results of measurements 

 made with different instruments. In the observations indicated 

 by an asterisk, the limit of discrimination has been taken to be the 

 point where the meshes of the network could be clearly recognized 

 by an observer who was not familiar with the object. As the 

 intensity of illumination is, of course, not altogether immaterial, 

 most of the determinations were repeated several times with clear 

 and with cloudy sky, and the mean values taken as the measure. 

 The distance of the axes of the wires is taken as the diameter of 

 the meshes, according to the method of calculation above referred 

 to ; it is, of course, equal to the sum of the diameter of one wire 

 and an interspace. 



Some of the data already given relating to the objectives of 

 Oberhreuser, Amici, Ploessl, Kellner, and Baader, are omitted from 

 the table, as they do not admit of comparison, nor are they of any 

 practical importance. To compensate for this, the enumeration 

 of results is enriched by a greater number of examples from the 

 recent publication of Otto Mueller ; 2 as he also made his measure- 

 ments as above described, his results can be compared with each 

 other as well as with ours. 



The objectives examined were, for the most part, of recent con- 

 struction ; some, however, were not so ; on this point therefore we 

 add a few special remarks. Both the objectives of Ploessl belong 

 to an old, large Microscope ; those of Merz, on the other hand, to 



1 Harting takes in these measurements the thickness of the wires and the 

 size of the interspaces separately. We do not consider this distinction to be 

 necessary for the test in question, since we have always obtained nearly the 

 same result with different network, in which the proportion of the thickness of 

 the wire to the size of the meshes varied between 1 : 5 and 1:2. 



2 Vergleichende Untersuchungen neuerer Mikroskop- Objective." Berlin, 

 1873. 



K 



