Some Ahhc Letters. 227 



angles of the common rows, one after the other, and among those 

 one set parallel to the axis of the scale. For that purpose you 

 bring the beam of light oblique in the direction of the axis of the 

 scale, until the pencils h, f (or as well c, e) come nearly to the 

 diameter of tlie aperture — in order to get as much light as possible 

 — and stop off a and g. The red [dotted] lines in the figure 

 indicate the arrangement of the diaphragm. 



As to your other question relating to the best position of the 

 diaphragm in your condenser, the principle for determining is very 

 simple (fig. 11). Let A be the objective, focussed to the plane E : 

 B the condenser beneath, D the diaphragm below the condenser. 

 Now the two systems A and B form together one system which in 

 every case will produce an image of the diaphragm. If D is out- 

 side the under focus of the combined system A and B, this image 

 will be a real and inverse image d, as assumed in the figure. Now 

 from the nature of optical images, all the rays passing from one 

 point of the diaphragm D, will cross in one point of the image d. 

 In your case, where it is necessary that the beams of light from 

 the different parts of the diaphragm (the different apertures) should 

 enter into different apertures above the objective, the image d 

 therefore should coincide with the edge of your prisms. If those 

 prisms with their lower edge, were exactly in the upper focus of 

 the objective A, the diaphragm D ought to be in the lower focus of 

 system B. Now in the higher powers the upper focus always will 

 be within the lenses : your prisms therefore, will be outside the 

 focus of A. The diaphragm therefore ought to be within, i.e. above 

 the lower focus of the condenser B. 



I presume that it will not be possible to bring the diaphragm 

 near enough for getting the coincidence of d with your prisms, but 

 in every case you will find the best position in the following way 

 experimentally : you adjust the objective to the plane of the object 

 and look down into the tube, without eye-piece. In moving the 

 eye you will observe a parallax between the images of the apertures 

 in I) and the lower edge of your prisms seen by reflection. The 

 best position of the diaphragm will be found if the parallax is 

 brought to the minimum, which is possible with your apparatus. 



I hope you will find intelligible my expositions and will 

 succeed in making all the experiments after having got the prepara- 

 tion announced. I will be very glad to hear from your results in 

 those experiments, and I shall take great pleasure in giving you 

 any further information you should want. 



Yours truly, 



(Signed) E. Abbe. 



