Wltat did our Forefathers see in a Microscope"? 437 



the single it was bathed in coloured fog, but the compound gave a 

 better picture of it than one would have expected. Of course, 

 chromatic aberration and spherical fog were there, and could be 

 easily found if sought out, but looking at the diatom in an 

 ordinary way these defects were by no means obtrusive. 



The minute hairs on the proboscis of the blow-fly were seen — 

 blunt and fluffy, it is true — but the Podura was quite beyond 

 either of them. No amount of words or of writing can convey to 

 another an idea of the quality of an image, therefore it will be 

 better to give a comparison with some modern apparatus, so that 

 any one may see for themselves the sort of image an attempt has 

 been made to describe. If any one will take a Zeiss aa and a 

 5 eye-piece, they will see an image which will represent, as closely 

 as the circumstances will permit, the utmost that could be made 

 out of any object by our forefathers with a compound dioptric non- 

 achromatic Microscope of the best kind. Of course, the brightness 

 and beauty of the image given by that remarkably fine semi- 

 apochromat will not be seen in the old instrument, but so far as 

 making out detail or structure is concerned, the two images will 

 be identical. 



With regard to Dr. Smith's catoptric Microscope, if we sub- 

 stitute for the aa the Zeiss 24 mm. apochromat, we shall see an 

 image probably not so very dissimilar to that given by Dr. Smith's 

 reflector. The detail resolved, and the achromatism in both would 

 be the same, but the reflector could not possibly reproduce the 

 brightness of image given by that beautiful apochromat. With 

 daylight illumination and only the concave mirror striie would be 



for the positive image is 10 in., while that for the negative is 8*8 in. If the 

 manipulator does not know the correct image, there is a way of finding it out, 

 thus : Place the top on the condenser, and increase the size of the stop to j\f, the 

 ground will now be dark, and, consequently, there will be only one point where 

 the lens is in adjustment, in this instance 9*2 in. Next remove the top lens 

 from the condenser, replace the |-in. stop without disturbing the lens adjust- 

 ment. The image now will hardly be so striking as either the positive or 

 negative images just described ; but it will be the true picture, the areolations 

 are not quite so black, the silex is not quite so sparkling white, nor of such 

 tenuity, but, if carefully considered, it will be perceived that the lens has been 

 adjusted upon the actual silex. If the former had been true positive and 

 negative images there would have been very little, if any, alteration of lens 

 adjustment required The presence of 1*2 in. of alteration in the adjustment is 

 an indication, to any one acquainted with the mere alphabet of microscopy, that 

 something is wrong. It will be asked, if the true image is found out by means of 

 dark ground illumination, why alter it, and why not show the diatom as it is 

 upon the dark ground ? A very natural question. The reply is that the valve 

 has some thickness, therefore with a lens of the aperture required to show this 

 diatom properly there will be much super-amplification of those parts which are 

 out of focus. This greatly interferes with the beauty of the image, and gives rise 

 to a mist, or fog, which the unskilled might take for spherical aberration. Upon 

 the semi-dark ground the mist is, of course, there all the same, but it cannot be 

 seen. This method of working upon a semi-dark ground is a very good one, and 

 will be found to be of much practical use even by a biologist in his rough and 

 ready work. 



