WITH THE MICROSCOPE. 183 



between black slag and that very hard constituent of some kinds of 

 steel, which remains so bright and polished after having been 

 acted on by cold diluted acids, as to look quite as black as the 

 slag by ordinary illumination. In fact, I may say that in studying 

 irons and steel such an illuminator is indispensable. 



23*. On Measuring the Angles of Crystals Goniometer. I have 

 already adverted to the principal methods of measuring objects, but 

 have not discussed the mode of ascertaining the value of the angles 

 of microscopic crystals in the microscope. The simplest instrument 

 for this purpose is one which \vas arranged many years ago by 

 Schmidt and known as Schmidt's goniometer. It consists of a 

 cobweb stretched across the field of an eye-piece, and capable of 

 being moved by an arm which passes round an accurately graduated 

 arc. The cobweb line is placed parallel to one face of the crystal, 

 the circle being moved round until the bar stands at zero. The 

 latter is then made to rotate until the cobweb is brought parallel with 

 another face. The number of degrees through which the bar has 

 passed marks the angle of the crystal. It is absolutely necessary 

 that in taking this measurement the crystal should be perfectly flat, 

 for otherwise a false angle will be obtained, pi. XLII, figs. 269, 270. 

 Dr. Leeson has proposed a more perfect arrangement for measuring 

 the angles of small crystals. PI. XLII, fig. 268. 



Those who devote themselves to mineralogical or crystallographic 

 investigations require special appliances for determining the optical 

 properties of refracting bodies and observing the process of crystal- 

 lisation in saline solutions, &c. Dr. Lawrence Smith, of Louisville, 

 U.S., designed an instrument specially for such purposes, which he 

 called the " INVERTED MICROSCOPE " (American Journal of Science, 

 second series, vol. XIV, 1852). The object-glass was placed below 

 the mica, quartz, or glass plate that carried the solution to be crys- 

 tallised, with the view of protecting the lenses from the corrosive 

 action of acid vapours, especially that of hydro-fluoric acid, which 

 also interfere with the definition of objects under examination. This 

 arrangement was improved upon and more fully developed in its 

 applications by Mr. Highley, who described the " Mineralogists 

 Microscope," figured in pi. XLIII, in the Quarterly Journal of 

 Microscopical Science, vol. IV, p. 281. It may thus be briefly de- 

 scribed with the aid of the figs. 271 and 272. The general distri- 

 bution of parts is shown in the first figure, when the instrument is 

 arranged for ordinary microscopical observations. Fig. 272 displays 

 the same in section arranged for optical investigations, and for 

 measuring the optic axis in crystals. 



On a central pivot screwed into a solid circular base rotates a 



