422 Transactions of the Society. 



necessary for the production of a photograph by the Microscope. The 

 reason of the cumbersome form of the photographic apparatus is, of 

 course, to be found in the use of the projection ocular with its great 

 focal length. If, instead of a projection ocular, you employ a second 

 objective, forming the- image of the first objective in the ordinary 

 way and then viewing that image as an object through a second 

 Microscope, the whole of the necessary apparatus may be contained 

 in a single draw-tube. On the table this evening there is a 

 Microscope of this sort fitted with a compounding draw-tube which 

 adds only 5 in. to the length of the instrument. It contains a 

 -i-in. for its second objective. In its present condition, with a 

 -^ oil-immersion for its principal objective it produces at a distance 

 of 10 in. from the stage negatives with a magnification of 400 

 diameters. The same magnification by means of a projection 

 ocular would necessitate a back focal length of at least 28 in.,, 

 and 28 in. of back focus implies mechanical contrivances for 

 operating the fine adjustment, and therefore, the horizontal posi- 

 tion and all the business of a photomicrographic outfit. Here 

 you have nothing more cumbersome than the ordinary Microscope. 

 What then, it may be asked, is the advantage which makes the 

 projection ocular in use so far preferable to the doubled objective 

 as to warrant the expense and trouble involved in the use of the 

 projection ocular ? Theoretically the one appliance cannot give 

 better resolution than the other, for we know by the Helmholtz 

 theory that repeated magnification does not impair the resolution of 

 the image, for image and antipoint are magnified together, provided 

 that no supernumerary diaphragm cuts down the beam which 

 carries the image from one focal plane to the next. But precisely 

 in this matter of diaphragms the projection ocular has a great advan- 

 tage over the doubled objective. For the projection ocular is placed 

 well forward in the beam, where the wave-fronts are crowded 

 together, and requires no lenses behind it. The course of the beam 

 is, therefore, perfectly clear and under these conditions the highest 

 magnifications can be obtained without sacrifice of resolution. The 

 use of a compounding draw-tube, on the other hand, presents many 

 difficulties. The first image fills the aperture of the second objec- 

 tive with very narrow and very widely separated beams of light, 

 and it is not altogether easy to avoid mutilating the more excentric 

 of these narrow beams by the edge of the aperture of the second 

 objective. The mischief of this mutilation of the marginal pencils 

 of light has been already explained and for this reason an image 

 highly magnified in this way, if no precaution is taken to prevent 

 mutilation, is little fitted for close examination although pictures 

 at comparatively low magnification may be made so with com- 

 plete success. 



