The Present Limits of Vision. By Dr. Boijston-Pigott. 185 
(3) Focus upon P, then tlie colours of Q and E will appear 
totally changed. 
Collecting these facts, it will be found that the colours of each 
of the three disks P, Q, E, do change their colours incessantly with 
every slight alteration of focussing. 
A most brilliant blood red, rose colour, lavender, purple and 
yellow green, may be readily obtained by mere change of focus : 
and if the number of disks be increased, and their distances varied, 
a gorgeous variety of colouring will be displayed, eclipsing the 
richest tints of the rainbow : provided sunlit disks be employed. 
The telescope, which is of the finest quality, will show the greatest 
variety of beauty in these coloured disks, one only — ^namely, that 
which is in the most nicely adjusted focus — appearing white. 
It may also be remarked that the quality of the telescope may 
be determined by the smallness of the change of focus, which pro- 
duces a coloured disk from a white one ; and the rapidity with 
which the disk goes in and out of focus. It is utterly impossible, 
in the nature of hght and achromatism, that brilliant objects in 
different planes of vision, at a greater or less distance from the eye, 
can be all in focus at once. Those that are nearer the eye than 
the true focal point will be of a different hue from those farther 
from the eye and beyond the focal point. If the telescope be under- 
corrected, the nearer disk appears ruby red and the farther a deep 
blue: with all manner of intermediate colours for intermediate 
brilliant points. The same holds exactly true for the microscope 
as for the telescope, only it is produced upon a very minute scale of 
focal distance. 
Apply then these facts to the microscope. Suppose a closely 
beaded object is illuminated transparently, and that several masses 
of beads, though close, lie really in different focal planes : these 
beads, if refracting, become brilliant disks, though excessively 
minute. For a perfect objective, only one set can be in the same 
mathematical focal plane. Suppose one of these to be Q, then 
those lying nearer to the eye, as P, will necessarily appear of a 
different colour from those beyond Q, as E ; and according to the 
variety of their situations, so must necessarily their colour vary to 
the eye of the observer. 
There are times, as already remarked, when, owing to good 
fortune in the various conditions of vision, appearances are developed 
which are often again sought for in vain. A lucky turn of the 
screw-collar; or of change in the length of tube; thickness of 
covering glass, and the best kind and colour of illumination, 
may reveal to the glance of the microscopist novel structures 
not perhaps easily caught again. During the astonishing though 
gradual advances made in the powers of the microscope, most 
persons who have watched its history and career during the last 
