WITH THE MICROSCOPE. 237 



unaffected portion of the prepared plate. Again, the thicker and 

 thinner parts of the same object may be exposed for different periods 

 of time, by which a uniform intensity may be obtained in spite of 

 the variable transparency of different parts. 



For the low powers the plane mirror, but for the |-inch objective 

 and higher powers some form of condenser is used, as a bull's-eye 

 lens, about 3 inches diameter. But for the finer forms of objects, 

 as diatoms, the bull's-eye lens is to be combined with a condenser 

 of the form proposed by him in April, 1861, for his binocular micro- 

 scope. This consists of a set of three plano-convex lenses varying in 

 diameter from about i|- inch to -i an inch, placed near to each other 

 with their flat surfaces towards the object. These combined possess 

 a very large angle of aperture. The small lens being made separable 

 from the others, a large field of illumination could be obtained for 

 the lower powers. 



32?. Brevet Lieutenant-Colonel Dr. Woodward's Method. This 

 will be a suitable place to introduce the plan adopted by Lieu- 

 tenant-Colonel Dr. Woodward, at the Army Medical Museum, U.S., 

 and which we quote nearly verbatim from the British Journal of Photo- 

 graphy for October i2th, 1866. "A camera is not used, a dark 

 room being found most convenient. The operating room has two 

 windows, through one of which just enough yellow light is admitted to 

 permit the movements of the operator. The lower part of the other 

 window is occupied by a shutter about fourteen inches high, on 

 which the blackened sash shuts down light-tight. In this shutter is a 

 round hole an inch and a-half in diameter, from the inner side of 

 which a brass tube of the same diameter projects into the room. On 

 the outer side of the hole is a rod about twelve inches long, on the 

 extremity of which the microscope mirror is duly centered. Two 

 steel rods attached by hooks to the mirror and passed through the 

 shutter, permit its position to be adjusted by a person standing 

 inside of the room, without opening the window. A Silbermann's 

 heliostat standing on a shelf just outside of the window, throws 

 the sunlight steadily upon the mirror. Within the room a frame of 

 walnut, ten feet long, is placed on a firm table perpendicular to the 

 window. The microscope stands on the end of this frame next the 

 window ; its mirror is removed, being replaced by that outside the 

 shutter. The microscope is placed in a horizontal position, and the 

 tube carrying the diaphragm or the achromatic condenser fits into 

 the tube projecting inward from the shutter, by which the sun's 

 light reflected from the mirror outside is admitted. A black velvet 

 hood covers the parts about the stage and objective of the micro- 

 scope, and thus prevents the leakage of light into the room. 



