On a Modification of the " Slit ''for Testing Angle. 21 
flakes " in such various and inconsistent localities — viz. in blood, 
urine, the brain, in tumours, and even in the breath — has hitherto 
aroused no suspicion of their true nature ; and it is only when we 
remember how few investigators have minds achromatic enough 
to enable them to see objective facts without subjective colouring, 
that we can offer a plausible explanation of this remarkable pheno- 
menon. Does not the delusion which, if I am correct, has thus 
entangled several eminent observers, form one of the most curious 
episodes in the history of medical microscopy ? and should it not 
serve as a warning to future generations of students ? 
Nevertheless, being always open to conviction, I hereby chal- 
lenge any devout believer in pigment-flakes to bring me an honest 
specimen of urine, or blood from any ordinary case of disease, in 
which can be demonstrated either pigment - flakes, pigmentary 
particles, or pigment-scales. 
Philadelphia, November 7, 1874. 
III. — On a Modification of the Slit" for Testing Angle. 
By K. B. ToLLEs, Boston, U.S.A. 
It is only within a few days that I have used the " sUt " devised by 
Mr. Wenham to cut off false light, as he says. Having always 
made the test of the verity of my measurements this condition, 
viz. that the object be in focus and in view with the extremest rays 
traversing the sector, there could remain no chance of my being 
mistaken. But in tracing a diagram to demonstrate the effect of 
aberration when the slit was used without cover and at the closed 
point, I hit upon a ready means of use of the slit to test angle with 
cover, and without, almost simultaneously. 
An ordinary glass object-slide, silvered on its upper surface, _ A. 
S, the slit cut through the amalgam (or silver), c, covering 
glass, with test diatoms in balsam between cover and slide, in the 
