184 Transactions of the Boyal Microscopical Society. 
groove, or a diflfraction compounded of the shadows of contiguous 
lines ? 
It is to be regretted that this great artist in minute engraving 
does not make some of the lines longer than others, so that the 
behaviour of a single line produced could be carefully scrutinized, 
as suggested by Mr. Broun. 
The limits of vision are also affected seriously hy the colours of 
bright objects, which are of necessity developed by their position in 
different focal planes, and particularly by accidental destruction of 
diffraction.* 
There are many persons who still strongly object to a variegated 
display of colouring in beaded objects ; those who have been long 
accustomed to purely achromatic glasses, such as were attempted to 
be made as the highest pride of the celebrated Andrew Boss, who 
succeeded in producing glasses so nearly achromatic, that an ordi- 
nary object, such as the Formosum, appeared of a very pale yellowish 
tint, approaching white, relieved by dark markings or striae. I re- 
collect the double striae of the Hippocampus were so displayed to 
me about thirty years ago by the famous Topping " with a 
quarter." And the attainment of this result with a quarter was 
regarded as a fine reward of manipulative skill. Well, if we compare 
these old glasses with the most splendid productions of the present 
day, the pale yellow and dark grey tinge have vanished in favour 
of a rosy pink and black. This change is highly significant. 
Higher and deeper planes develope colours, forming the colour test 
formerly described by me ; whilst the finest focal plane presents 
black lines combined with a pale rose-pink blush. 
Turn we again to the telescope. This instrument beautifully 
illustrates the optical qualities of the microscope. Let E 0 P Q K be 
the axis of the telescope directed to three brilliant minute disks, 
P, Q, E, so arranged that each disk can be seen in the same field of 
view. 
' E O ', P Q R 
(1) Let the telescope be focussed on the middle disk Q, then, if 
the telescope be of the very finest quality, the disk Q will appear 
white ; but the disks placed within and without the focus will at 
the same time appear of totally different colours, according to their 
distance from Q. (This is a novel test of the corrections of the 
glasses.) 
(2) Focus the telescope on E, the most distant point, then the 
disks Q and P will totally change their colours, according to their 
distance from P. 
* " It is not impossible," says Helmlioltz, " that by some fortuitous overlapping 
of images, objects of still smaller dimensions might occasionally be seen," i. e. less 
than half a wave-length. 
