MICROSCOPICAL DIAGNOSIS. 



aberrations, but by the use of the two kinds of glass already men- 

 tioned they are nearly obviated. Objections thus made are said to 

 be "corrected." 



An objective is over-corrected when it shows a. bluish border 

 and under-corrected when it shows a reddish border, and as the blue 

 color is the more agreeable to the eye, so all objectives are slightly 

 over-corrected. While defining power shows the outline of a speci- 

 men well, resolving power enables the observer to detect the most 

 intricate structure on its surface. Every practical worker is aware 

 that this quality depends upon the "angular aperture" of the glass. 

 Given two glasses equal in all respects but that of angular aperture, 

 and that glass having the widest or highest angle will give the best 

 resolving power. Professor Abbe says that the maximum attainable 

 resolving power, with an angular aperture of 180 should separate 

 118,000 lines to the inch. Yet just at this time Mr. Ed. Bausch, of 

 the firm of Bausch and Lomb, of Rochester, N.Y., has completed an 

 one-eighth inch objective of high angular aperture which, in Dr. Up 

 de Graff's hands has resolved the band on Fasoldt's test-plate 152,- 

 400 lines to the inch. Perhaps Fasoldt with his 1,000,000 lines to 

 the inch may not be able to conquer the whole world of working 

 microscopists after all! 



For the discussion of the vexatious question of angular aper- 

 ture, the reader is referred to Frey, Carpenter, Smith, and to the 

 many articles in various journals, on that subject The usual defini- 

 tion is this: The angle of aperture is the angle formed by two lines 

 extending from the point in focus to opposite sides of the aperture 

 of the objective. 



Working distance is the distance between the front glass and 

 the point in focus. While even in the lower powers working dis- 

 tance is a desirable quality, yet in the case of the higher powers 

 it is of essential importance. It is here that the immersion systems 

 are so valuable, giving an increase in this distance. Penetrating 

 power enables us to look deep into the structure of an object. 



Carpenter says it may be defined as consisting in the vertical 

 range through which the parts of an object not precisely in the 

 focal plane may be seen with sufficient distinctness to enable their 

 relations with what does lie precisely in that plane to be clearly 

 traced out. Professor Abbe says that, theoretically, the plane of 

 construction remaining the same the "penetration" of an objective 



