328 NOTES AND MEMORANDA. 



the case of uniaxial crystals. I have not yet been able to thoroughly 

 ascertain the laws which govern these special peculiarities, and no 

 kind of explanation has yet suggested itself either to Professor Stokes 

 or myself ; and therefore it appears to me undesirable to enter more 

 fully into the question, which relates more to the mathematical theory 

 of light than to practical mineralogy. It may, however, be well to 

 say that the discrepancy to which I refer is in the ratios of the values 

 of the real and apparent indices." 



Mr. Sorby gives an illustration of the application of the method to 

 the identification of doubtful minerals, in the case of certain crystals, 

 which he determined to be an unusual secondary form of calcite. 



Improved Achromatic Condenser. — Messrs. E. and J. Beck have 

 introduced a modification of the achromatic condenser, in which a 

 series of combinations of lenses are made to revolve excentrically, so 

 as to be brought consecutively into combination with a lower fixed 

 series of lenses. The apertures vary from 40° to 170'', and two of the 

 revolving combinations are truncated and blackened, so as to stop out 

 the central rays to the limits of GO'^ and 120°. 



The latest addition to the instrument consists of the application 

 of a revolving diaphragm, with various sized apertures beneath the 

 entire combinations. 



Seller's Mechanical Microtome. — Dr. Carl Seiler, of Philadelphia, 

 is the inventor of an apparatus for enabling the knife, in cutting 

 sections, to be carried through the tissues with an even motion and 

 at the same inclination — a necessary point to ensure success, but not 

 so easy as might be imagined, because the hands usually are not sufii- 

 ciently steady without a great deal of practice. 



It occurred to Dr. Seiler, therefore, that if the knife could be 

 rigidly fastened to some apparatus by means of which it could be 

 moved over the well of the microtome in the same manner that the 

 hands move it, sections of any size and thinness could easily be made, 

 even by an unpractised hand ; and after some experimenting he con- 

 structed, with the aid of Mr. Zentmayer, a mechanical microtome 

 which proved to be all that could be desired. 



It consists of two rigid, parallel arms of metal, which at one end 

 revolve on pivots attached either to the microtome itself, or to the 

 table to which the microtome is to be clamped. On the other end of 

 these arms are fastened revolving clamps which hold the knife, the 

 edge of which, when in position, rests upon the glass plate of the 

 microtome. The handle of the knife is removed, so as to prevent a 

 slipping and hindrance to the motion of the knife, but can be easily 

 attached by means of a screw, for the purpose of stropping. 



When in position and ready for cutting, the knife is pressed upon 

 the glass plate, and a slight side-motion is given to it by the hands, 

 which causes it to pass through the tissue, and cut a thin, even section 

 without difficulty. With this apparatus he was able to cut a thin 

 section of the leg of a five months' fcetus, from the knee downward, 

 including the foot, the section measuring 2 inches in length by f inch 

 in width. Several mechanical microtomes have been constructed by 



