SUBSTAGE CONDENSERS, ETC. 93 



wondered how its use has ever been adopted in this and other countries in the manner 

 it has, but possibly the words of Dr. DalHnger sufficiently explain it when he says : 

 " The fact is that a large part of the admiration that has been expressed for this 

 condenser has resulted, not from a comparison of its results with those of other high- 

 class achromatic condensers, but of images obtained without any substage optical 

 arrangements at all, placed in contrast, with the results obtained by using this 

 condenser against the same objective when used without its aid." 



A microscope without a condenser is not a microscope at all in these days of perfec- 

 tion, and those who declare that with medium-power photo-micrography no condenser 

 at all is required are mistaken, that is, if they desire to obtain perfection of result. 



Condensers, theoretically, should be of the same N. A. as the objective, even to 

 I "40, but, practically, it is not of so much service if they are, as no objective — 

 excepting when employed on bacteria — that we are acquainted with will stand a solid 

 cone of light from the condenser the same size as its own N. A. 



If one should be so constructed we should look for much greater perfection of the 

 final image, but at present even the Zeiss, and Powell and Lealand apochromatic 

 objectives will not bear more than about two -thirds of the illumination of the back 

 lens, as seen down the tube of the microscope on removing the eye- piece, when 

 employed on objects in general, such as diatoms. It will be thus understood if we 

 could obtain a really perfect condenser of about ro N. A., it would practically 

 illuminate, at its full solid cone aperture the amount required to" about fill a 1-40 N. A. 

 objective so far as necessary ; and observers who have tried a condenser of higher 

 N. A. than this, without prejudice have expressed their belief that inasmuch as the 

 higher N. A. has to be cut down to fill only two- thirds of the objective, the advan- 

 tage gained by the high- angle condenser must be lost. And although some who read 

 this may doubt the truth of these remarks, they seem based on careful observation, 

 and after all are only rational deductions. Let two photographs of subjects other 

 than bacteria, for example, be tried, as we have known, one with a 1-35 N. A. con- 

 denser cut down to the necessary aperture, and the other when using a i -o N. A. 

 condenser with its full aperture ; the difierence between the results, if using direct 

 light and solid cones, have appeared to be indistinguishable. 



But when employing the microscope on bacteria — by which is meant when 

 photographing them at high magnification — a full cone of light must be employed, 

 and here it may he found of service to employ a high N. A. condenser, because it 

 shortens exposure, and, in some cases, may sharpen the general aspect of the bacilli. 

 If a small solid cone be employed, by closing the iris, a luhite diffraction riyig will be 



