248 HOW TO WORK 



what is called "turning out." The coarse or rack adjustment is left 

 for focussing. By means of a lever arm at one end, clamping the 

 milled head, and at the other connected to a long rod resting at the 

 side of the apparatus, a very delicate movement is obtained. The 

 fine adjustment is left to regulate the compensation required between 

 the chemical and visual foci, and to mark this more easily, a " dial 

 plate of card " is attached to the body of the microscope, whilst a 

 wire which at one end is bent to clip the milled head of the fine 

 focussing screw, is at the other used as the index point for the divi- 

 sions of the card. 



The condenser recommended is a 3-inch focus bull's-eye lens, 

 with its convex side to the source of light, and in conjunction with 

 this the objective next below the one in use. Oil lamps, oxy-calcium 

 and magnesium lights have been used, but the last is preferred, and 

 the wire to be burnt in preference by successive flashes. To secure 

 the point of light being in a proper position " a small telescope 

 upright, of brass, regulated by a screw, is fixed to a block adapted 

 to slide in the support common to the microscope and light ; at the 

 apex of the brass upright is fixed a small tin gutter or pipe of suffi- 

 cient capacity to admit the wire easily and diverted down at an angle 



of 45-" 



A moveable stop with a pin-hole aperture is recommended to be 

 used in the preliminary arrangement to secure the exact position ; 

 about i of an inch of the wire is exactly opposite the pin hole. 



The camera is set to certain lengths so as to give images of the 

 objects of a definite size. The " turning out " consists in actually 

 testing each objective for the number of turns or parts of a turn of 

 the fine focussing screw by means of the dial card, to make the cor- 

 rection for the actinic focus. In the high powers this may be almost 

 disregarded, and the same result may be obtained by withdrawing 

 the focussing screen to the point where by trial the true actinic focus 

 has been fouud. In the Popular Science Review, No. 22, 1867, 

 will be found an illustrated memoir by Dr. Wilson, in which full 

 particulars have been given, and from which the foregoing remarks 

 have been taken. 



The time of exposure for wet collodion plates varies, increasing 

 according to the colour of the object, and its enlargement; 50 

 diameters and a tolerably light object may need ten minutes with the 

 oil-lamp. By placing a small vessel of warm water in the camera, to 

 keep the collodion plate moist by its vapour, Drs. Abercrombie and 

 Wilson have exposed plates forty minutes with success. Some of 

 the prints from these gentlemen's negatives are remarkably good ; 

 they possess a peculiar delicacy in the half-tones and shadows, 



