176 Miscellaneous. 



them fatiguing and hurtful to the eyes. To overcome this ob- 

 jection, we were led to experiments in making micrometers by the 

 aid of photography, which have resulted in success. The steps of 

 the process are these : — 



1st. A scale of 100 heavy Indian-ink lines, about ^ of an inch 

 apart, are drawn on a dead white surface of Bristol board. The 

 lines marking every ten divisions are 6 inches long and extend 

 one inch each side of the scale ; those marking every five divisions are 

 5 inches long, and extend half an inch beyond the scale ; the re- 

 maining lines are 4 inches long. 



2nd. By photographic process for copying engravings, a negative 

 is taken, on which the scale equals about 2 inches in length, and is 

 intensified by mercuric chloride and potassium cyanide. 



8rd. With a copying-camera and lens for taking transparent 

 positives for the magic lantern, a transparent positive of this nega- 

 tive is taken on micrometer glass, reducing the scale to the length 

 of half an inch. In this the lines are ^i-jj of an inch apart. After 

 intensifying, washing, and drying, a cover of thin glass is cemented 

 on with Canadian balsam, and the slide cut to fit the slit in the 

 micrometer eyepiece. It can also be mounted with a spring and 

 micrometer screw, like Jackson's micrometer. In our micrometer 

 the lines appear to stand out in relief, and are jet-black, while the 

 spaces between them are translucent enough to admit of the accu- 

 rate measurement of the details of minute algae and fungi to the 

 r^j^ of an inch. 



Regarding the goniometer : — 



1st. A circle, about 18 inches in diameter, is drawn with Indian 

 ink, divided into degrees. The centre is indicated by a dot, and 

 one diameter is drawn. Every five and ten degrees are indicated 

 by longer lines than those indicating single degrees. Every ten 

 degrees of each quadrant is numbered, from to 90. 



2nd. A negative 2 inches in diameter is taken by the process 

 referred to above ; and from this a transparent positive is taken on 

 a circle or micrometer glass cut to fit the tube of the microscope. 

 It is covered with a circle of thin glass cemented with balsam, and 

 mounted to fit the tube at the focal point of a positive eyepiece. 

 A cobweb is drawn across the diameter of the lower lens. When a 

 crystal is to be measured, the stage is moved till the apex of the 

 angle coincides with the centre of the goniometer, and the diameter 

 with one side. The eyepiece is now turned till the cobweb cross- 

 ing the diameter at the centre coincides with the other side of the 

 angle. Now the number of degrees of the angle can be read at the 

 circumference. The advantage of this over the ordinary microscopic 

 goniometers is, that in ours the angles of the crystal and the de- 

 grees of the goniometer are on the same line of sight within the 

 tube of the microscope, while in the ordinary goniometer the de- 

 grees are marked outside the tube. The photographic processes by 

 which the above are made can be learned by consulting any of the 

 standard works on photography, under the sections that treat of 

 copying engravings and taking transparent positives. — SllJiman's 

 American Journal, Dec. 1871. 



