478 77/ /v AMKRK AX XATURALIST [Vol. LV 



hesitate to spend some time fti tlie preparation of such tables. 

 It is, of course, natural that ditroiTueos in the source of light, 

 optical equipment and r;iy-filt('rs necessitate the preparation of 

 new tables. Our own outfit beiim- one of the standard Bausch 

 and Lomb apparatus with a Zeiss microscope and Cramer ray- 

 filters, it occurred to me that those possessing a similar e(iuip- 

 nient might be spared the tediousness of preparing an exposure 

 t;dile if our table of exposure factors were published with such 

 iiisi I'uctions as would permit of identical arrangement of the 

 apparatus. Moreover, it will be remembered that these factors 

 are dependent upon the source of light, substage position, mag- 

 nification, and numerical aperture of the objective, and that 

 other conditions being identical, exposure varies as the square of 



A word must be said about the use of the substage condenser. 

 The position of the condenser indicated in the table is such as to 

 give the greatest detail without apparent bad effect on definition. 

 As is well established, the depth of focus in a microscopic ob- 

 jective depends upon its numerical aperture. The ureter the 

 latter, the smaller the depth of focus. Since definition increases 

 with numerical aperture, one has to sacrifice the one or the other. 

 From a practical point of view depth of focus is often more 

 desirable than perfect definition. The numerical aperture of an 

 objective may be conveniently cut down by increasing the dis- 

 tance between the substage condenser and the objective. This 

 can be done with safety only to a certain point, which 1 called 

 the optimum, and beyoinl whieli definition is visibly impaired. 

 I hope to be able later to return to this subjei^t in greater detail. 



magnifying j)ower of an objective, the sooner tlie 0{)tiHuim will 

 be reached. With other words, the substage may be lowered a 

 great deal more in low-power than in high-power objectives. 

 In the following table the position of the substaize is indicated 



as it Will gn. Ill the Zeiss inicrophotographic stand ilie substage 

 rotwlrti^cf lias a iiiiitien.-al aperture of 1.40, and when jilaced at 

 /en. the surf.irr uf its iipp.-r lens is still O.S mm l.elnu the 

 surfnceof the mieroscnpi.- table. 



The .-ource of li-ht fnr ul.irh the f.il.lM ..f .xpnMPr tartors 

 holds good is the liaiisrh ;i,id \.nmh .Mirrnpn.jrrrni' \n. 4:{01 

 with hand feed arc lamp and I'heostat for 4..") ainpei'es. 110 volts, 



