ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 625 



Either or both of the projecting portions of the trough can be shut off 

 from the main receptacle by sliding hatches (not shown in the fig.) 

 if necessary, and the part containing the body of the frog can bo 

 covered with glass or a vulcanite lid. Should it be desired to observe 

 the effect of gases or of heat or cold, the required gases or warm 

 or cool air may be conducted through the body chamber by means 

 of the two small tubes seen projecting from the front and sides 

 respectively. 



The frog to be observed is placed either ventrally or dorsally as 

 may be required, and is held by means of loops of thread passed 

 round the arms and then led through screw-eyes and clamped up. 

 The thighs are held by a pair of stocks, which, by means of a sliding 

 upper half, can be adjusted accurately to the limbs without causing 

 constriction ; and the webs are spread out by pinning loops of thread 

 tied to the toes. 



Visibility of Ruled Lines.* — C. Fasoldt writes, in regard to the 

 •note by Professor W. A, Rogers, which appears at p. 439 of vol. iii. 

 (1883), that " there are some statements which do not agree with my 

 experience. I find that lines properly ruled on glass are similar to 

 graven lines ; they are smooth, clean cut, having a definite shape and 

 depth. Such lines are always visible in the Microscope, and central 

 or oblique light will show the bottom of each cut as a dark or 

 coloured line, plainly visible, and requiring no graphite or other 

 foreign substance to indicate it. The Microscope is the test for a 

 properly ruled line. The mechanical elements (pressure, &c.) enter- 

 ing into the process of ruling are not at all evidences that lines have 

 been properly ruled. The slightest accident to the point of the 

 cutter, or the surface of the glass not being perfectly clean, will spoil 

 a line ; that is, produce a scratch which cannot be satisfactorily 

 illuminated in any light. Well-ruled bands of lines, 70,000 or 

 80,000 to the inch, are visible in the Microscope with central light ; 

 and with a Smith vertical illuminator (giving central light), I have 

 seen 100,000 lines to the inch. As these individual lines have a 

 width of about 1/200,000 of an inch only, it follows that the difficulty 

 is not to see such a narrow line, but to eliminate the diffractions 

 which tend to blur the image in the Microscope, and so prevent the 

 resolution or separation of the lines in a band of them." 



Mercer's Photomicrographic Camera.! — Dr. F. W. Mercer has 

 devised the camera shown in fig. 106. It consists of a box of light 

 wood A, a cone of light metal B, a tube which takes the place of the 

 ordinary draw-tube of the Microscope, C, and the frame carrying the 

 ground glass and plate-holder, D. The tube C is fitted to the cone 

 B, so that it may be withdrawn for the insertion of an eye-piece or 

 amplifier. To the box A is attached a brass strap a, the lower end 

 being slotted to admit the passage of a binding screw secured to a 

 button h, fastened to the arm of the stand. As soon as the object 

 is coarsely focused upon the ground glass the cone and its tube are 



* Scif.ntific American, xlviii. (188.S) p. 341. 



t ' Photography ' (Chicago), i. (1884) pp. 9-10 (I fig.). 



