100 SUMMARY OF CURRENT RESEARCHES RELATING TO 



except for a portion of the lens surface, and thus a lens will be produced 

 having a number of foci. In fig. 14 is shown an achromatic lens formed 

 by grinding the outer surfaces of the blank, the uniting surfaces between 

 the two layers being properly curved in forming the shell. 



(5) Microscopical Optics and Manipulation. 



Microscopic Measurements.* — A correspondent, J. D., in the 

 English Mechanic, points out the too frequent omission of information 

 as to the real sizes of objects delineated in book-illustrations of micro- 

 scopic objects. Even when a size is given (e.g. X 200), such a state- 

 ment is incomplete unless the particular combination of the eye-piece 

 and objective used is stated. The writer suggests that it might be 

 desirable to give the magnification obtained in microns (e.g. 100//.). 

 He also emphasizes the convenience of the squared eye-piece micrometer, 

 and points out that it may even be used for angular measurement. If 

 the left-hand bottom corner of a group of squares be regarded as the 

 centre of a circle, radii may be imagined intersecting the horizontal and 

 perpendicular lines where they cross one another. Taking the bottom 

 line as radius, the tangential perpendiculars may be expressed as decimal 

 fractions of this radius, from which decimals the included angles can be 

 found by inspection in any table of natural tangents, and other angles 

 thereby estimated. On the same principle the angles round any point 

 of intersection, within the group of squares, may be easily determined 

 and tabulated, if likely to be of use ; and for angles above 90°, the 

 choice of a suitable centre, conjoined with a little additional calculation, 

 is all that is necessary. 



(6) Miscellaneous. 



Gage's Microscopy.t — The Microscope : an Introduction to Micro- 

 scopic Methods and to Histology, by S. H. Gage, has quite recently 

 reached the loth edition. The"" work has been practically re-written, 

 and though retaining the well-known features which have rendered it 

 so popular and valuable, a vast amount of information as to recent 

 advances and improvements in Microscopy and Technique has been 

 added, more especially in the direction of Histology. To enter into 

 detail as to its merits would nowadays be superfluous, but it is per- 

 missible to state that this text-book will be found of the greatest service 

 to teacher as well as student, whether they be of the " brass and glass " 

 or " bug and slug " tendency. 



Cholesterol, Fluid Crystals, and Myelin Forins.:}: — C. Powell 

 White draws the following conclusions from facts ascertained after many 

 complicated experiments. The potassium salts of " oily " fatty acids can 

 be obtained in a crystalline fluid condition, which probably represents 

 a hydrate of the salt. In aqueous solutions of these salts (where the 

 corresponding acid is insoluble in water) cholesterol and some other 

 substances give myelin forms, which are due to variations in surface 



* English Mechanic, lxxxviii. (1908) p. 356. 



t Comstock Publishing Co., Ithaca, New York, 1903, 345 pp. (258 figs.). 



X Med. Chron. (1908) pp. 1-19. 



