LT.-COL. GIFFORD, J3K. W. ROSENHAIN 89 



I can only state the fact that if you get the same magnification 

 with, say, a J-inch lens and a long camera that you were getting 

 with a 1-inch lens and a short camera, you will hardly get more 

 flatness of field in the one case than in the other. 



There is just one other point, and that is that in all the photo- 

 micrographic apparatus which I have seen and possessed, there is 

 one fault which seems to be inseparable from the instruments, and 

 that is, with an extended camera, the connection between the operator 

 and the focussing portion of the microscope, especially with lateral 

 focussing milled-heads. These are extremely convenient, no doubt, 

 for bench work in the laboratory, but for ordinary purposes of photo- 

 micrography it is extremely difficult to connect them satisfactorily 

 with any form of extended focussiug arrangement, and in any photo- 

 micrographic apparatus which may be put forward that point should 

 certainly be kept in mind. The most efficient contrivance, I think, 

 that I have ever seen — and the hint may be of use to some here, 

 perhaps — was that of my friend, Dr. Neuhauss, of Berlin, who was 

 well known as one of the very first photoinicrographers in Germany. 

 He simply carried a straight arm down from the axial focussing head 

 of the fine adjustment, and attached a string to the lower end, with 

 a weight on one side and a drum on the other, and so he managed 

 to get his focussing fairly accurate. In conversation with Dr. Czapski 

 once, when he came to see me, I pointed out to him that I found 

 it impossible to get accurate focussing without tapping the bench, to 

 make sure that the last adjustment was as delicate as possible, and 

 he said, " Oh, that is quite the regular way for giving the final touch 

 in delicate measurement " ; so that I presume I had not gone very 

 far wrong. 



Lieut. =ColoneI Gifford : I have in my possession two of these early 

 Zeiss lenses. The late Mr. C. Lees Curties procured them for me 

 at a very early period. One is marked No. 2, and is a 6 mm. of 

 0.95 N.A., and the other is a 3 mm. of 1.40 N.A., and is* marked 

 No. 34. Neither of them have suffered in the slightest, and I use 

 them to-day as well as I did originally. On the other hand, I 

 have Powell objectives. One of them is a 1/10 of 1.5 N.A. — a very 

 large aperture indeed — and the other is 1/20 of the same N.A. The 

 1/10 became entirely obscured about two or three years ago. That, 

 however, has been renovated by the present Mr. C. Lees Curties. 

 The other one, the 1/20, has stood all through. At the same time, 

 if you compare the two makers, T am afraid we must prefer the Zeiss. 

 Eoth lenses which I possess of that make are simply perfect ; I sup- 

 pose they could not be quite perfect, but they are as perfect as they 

 possibly can be. They stand any power you like to apply to them. 



Dr. W. Rosenhain, F.R.S.: I want to draw attention to one 



particular point about the discussion which has impressed itself upon 

 me in listening to it, and that is that there seem to be two totally 

 distinct questions being discussed in a rather confused manner. The 

 one is the question of establishing a commercial and industrial pro- 

 duction of microscopes by mass production. This is, no doubt, a 

 very excellent and valuable industrial step, with which, of course. 



