ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
103 
tion of the picture is composed of three lines drawn at 60° to one 
another, the third line passing through the apices of the rhombs. I will 
next project a false diffraction ghost of P. formosum, showing intercostal 
dots (fig. 16). These were produced in precisely the same manner as 
the others. The focus, you will notice, is only slightly within the true 
focus. The greater the aperture of the objective used the less out of 
focus the object requires to be in order to produce the intercostals. 
Now I have shown you the three degrees of diffraction ghosts ; these 
are all produced, and can only be produced by the small cone. It 
cannot be wondered that Prof. Abbe and his exponents say that “ whether 
for example, P. angulatum possesses two or three sets of striae, whether 
striation exists at all, whether the visible delineation is caused by 
isolated prominences, or depressions, &c., no Microscope, however 
perfect, no amplification, however magnified, can inform us.”* 
Again, we read “ that every attempt to discover the structure of 
finely organized objects - as, for instance, diatom-valves — by the mere 
observation of their microscopic images, must be characterized as wholly 
mistaken.” And again, “ The interference images of minute structure, 
however, stand in no direct relation to the nature of the object, so that 
the visible indications of structure in a microscopical image are not 
always or necessarily conformable to the actual nature of the object 
examined.” 
The explanation of all this is that Prof. Abbe takes cognizance of 
one kind of image, and that one a diffraction ghost, and it is perfectly 
true that so long as you are dealing with diffraction ghosts you cannot, 
for certain, determine the nature of the structure you are observing. 
At different foci when a small cone is used there are different 
images, and without a priori knowledge it is impossible to determine 
the correct focus, and consequently the true diffraction ghost. Now it 
is the function of the condenser to put an end to all these difficulties ; 
it enables you to illuminate by means of a wide-angled cone, and then 
you have a true image at one definite focus, and at any other focus 
there is no image at all to confuse you. 
Of course it must be understood that when the structure is very 
fine, and the spectra are diffracted through great angles, your widest- 
angled cone really becomes a narrow one in relation to that structure ; 
and then you are obliged to make the best you can with diffraction 
ghosts. But there is, on that account, not the least reason why, for 
all coarser structures, you should not have a true diffraction image by 
means of a large cone instead of either a true or false diffraction ghost 
by a small cone. 
Eventually our diffraction ghosts with very fine structures and 
wide-angled cones may through increase in the apertures of our 
objectives and improvements in our condensers, be changed to true 
diffraction images. 
Prof. Abbe’s last paper takes account only of small differences 
between very similar images, and ignores altogether the enormous 
differences due to the union of different orders of spectra and the 
exclusion of others. He is in fact straining at the gnat and swallowing 
the camel. In his paper he disregards the possibility of getting (to 
* xiv. (1875) p. 220. 
