786 Botanical Microtechnique 



that has sexcral components and an E. F. of If) mm., -will give an 

 image ol the same size as a simple lens of 16 mm. focal length. The 

 E. F. should not be confused with working distance. 1 he equiAalent 

 focus decreases as the magnification increases. The experienced 

 worker is in the habit of speaking of an obiecti\e as a 4-nnn. objective, 

 for instance. For class use it is much better to speak in terms of 

 magnification, which is 43x in a certain 1-imn. objective. In the 

 past the manufacturers have paid undue attention to computing their 

 objectives so that the equivalent focus is an even number, and a series 

 of objectives will have the equivalent focus in the orderh progression 

 16, 8, 4, 2 mm., etc. A magnification may turn out to be some awkward 

 fractional ninnber like 3.2 or 5.1. A more practical series would be 

 a sequence of magnifications such as 3, 5, 10, 20, 40. 60. etc. There 

 is a trend toward the use of the latter system. 



DEPTH OF FOCUS 



A minute body or a very thin section has thickness or depth. If 

 a deep cell is being viewed \\ith a lOx objeciixe and the lens is 

 focused on the upper wall of the cell, the bottom wall mav also be 

 in focus. If a 45 X objective is focused on the top wall, the bottom 

 wall may be completely out of focus and practically invisible. If the 

 lens is focused on the bottom wall, the top wall becomes obscured. 

 The vertical extent of the zone of sharp focus is known as the depth 

 of focus. Depth of focus decreases as the magnification increases, 

 although magnification is not in itself the determining factor. There 

 are mathematical limits to the dejMh of \iew encompassed by a 

 given objective. Magnification and other factors being equal, objec- 

 tives of the several manufacturers have the same depth of focus. 



RESOLVING POWER 



Resolving power is that projjei t\ of a lens ^vhich makes possible 

 the recognition, as distinctly separated bodies, oi objects that are 

 exceedingly close together, oi subtended b\ a small \'isual angle. I he 

 simplest illustration of resolving power is the visibility of double 

 stars. Although ihe two stars may be sejiarated b\ a vast distance, 

 the visual angle reaching the eye is very small, and the stars appear 

 to be close together. Man) indixiduais can see but one star. Othei 

 persons, whose eyes have better resohing power, can see the txvo 

 stars distinctly. Applying this prin(ij)le to the microscope, a lens of 

 poor resolving powei will show a slender chromosome as a single 

 thread, whereas a lens of good resohing power will show the chromo- 



