37 



by a belt with the micrometer screw; furthermore, this exerted a slight 

 pull on" the microscope tube that rendered focusing very difficult; we 

 overcame our difficulties by first placing four adjustable brass pillars 

 under the microscope bench; the bench was now held down to the rods 

 by the binding screws and its distance fi'om the table was made abso- 

 lutely the same by the brass supports; ordinary sliding of the camera in 

 changing its length or putting in and taking out the plate holder does not 

 in any way damage the focus. To brace the microscope tube against 

 the pull of the focusing belt we supported it two and a half inches behind 

 the milled head of the micrometer screw by an adjustable brass pillar 

 reaching down to the camera table. Since making these additions we 

 have not lost a single plate by change of focus. This result can be 

 brought to pass in other ways, perhaps, but this is one good way and for 

 the following reason is, I believe, the best way: We have fastened also 

 to our camera table a brass rod inside of a brass tube, each provided at 

 the focusing plate end of the camera with milled heads and at the 

 microscope end with separate belts passing around the grooved heads that 

 control the moveable stage, so that the operator six feet away can system- 

 atically search a field over, that is three-eighths of an inch in diameter. 

 This is a convenience that comes near to being a necessity; it makes high 

 power work as controllable and as speedy as low; it turns drudgery and 

 annoyance into a pleasure; any one who ever undertook to center an object 

 by giving directions to an assistant at the microscope must know its 

 value. If an object is out of the field, finding it is hopeless in the old way; 

 it is perhaps enough to say for our arrangement that it enables one 

 person to do quickly and exactly what otherwise requires two at a cost of 

 much time, labor and patience. The downward pull on the stage is 

 counterbalanced by an adjustable brass support immediately under the 

 controlling heads of the stage. 



MAGNIFICATION. 



The linear magnifications possible range from six and a half with 

 the 70mm objective without an eyepiece to .5,500 with the 2mm objective 

 and an 8 eyepiece. The following table shows the magnification at vary- 

 ing lengths of the camera with a few combinations. They were deter- 

 mined in every instance by measuring on the ground glass the projected 

 image of a stage micrometer. 



