152 Transactions of the Boyal Microscopical Society. 
may be descried. The use of a good prism as a reflector gives a 
pure beam greatly superior to a quicksilvered plane mirror. On 
one occasion the subtle delicacy of the interference-waves of 
light was thus revealed, for the whole of a series of magnificent 
phenomena was wholly obliterated by substituting a plane ordinary 
mirror for the prism.* 
In the ' Proceedings of the Eoyal Society,' the appearances of a 
minute miniature of the solar disk, republished in this Journal, are 
so fully described, as dependent upon the quality and condition of 
the objectives employed, that I may be excused the trouble of re- 
capitulation here. I proceed therefore to treat of the miniatures of 
illuminated globules of mercury. These may be dismissed with 
the general statement that the symmetry, beauty, and fineness of 
the difiraction rings are severe tests of the objective. And 
finally : 
These rings are either wholly above or wholly below the focal 
point with a bright haze on the other side; or else equal and 
similar on both sides of the focal point ; or altogether ill-defined. 
The experimenter will soon ascertain for himself these various 
appearances. I therefore pass on to the effects of olliquUy. 
If the miniature-forming objective be slightly inclined, so that 
its axis has a few degrees of obliquity, a new order of effects dis- 
play themselves of extraordinary beauty and complexity. Sun-lit 
globules suffice ; but of course a minute focal disk of the sun in 
miniature gives more brilliant effects. 
The conditions of over or under correction are beautifully seen, 
and the best possible adjustments still give very pecuhar forms, 
some of which are shown in the accompanying drawings. 
The most extraordinary of all is the curiously winged butterfly 
form : . the cometary, double vase, and conical diffractions are well 
worth development. These very beautiful solar phenomena, as 
shown by a miniature of the illuminated globule, vary their 
exquisite forms according to the quality of the glasses, and 
according as they are over or under corrected,, and according to 
* The same destruction of beauty and symmetry occurs with a badly con- 
structed screw-collar, whose movements decentre the component lenses : a very 
little error in this mechanical adjustment produces huge derangement. Better 
is a glass of lower aperture without such source of error, than a large aperture 
and loose adjustment. . Indeed, a trifling difference of thickness in the glass 
cover introduces much milder error than the shake of the adjusting screw causing 
central displacements. Besides, if the glass be a little thicker than before, a 
slight extension of the draw-tube will compensate for thickness with extreme 
nicety. Lengthening the tube " over corrects," whilst a thicker glass intro- 
duces " under correction." In the case of telescopes with fixed glasses, I have 
never been able to persuade an optician to clean old glasses and reset them : it is 
almost always a total failure : nothing is more delicate and difficult than exactly 
centring the lenses of a telescope, an instrument which seldom magnifies more than 
500 times. How much more difficult then must be a movable construction of the 
many lenses of a microscopic object-glass, which is used for much higher powers ! 
