and Laboratory Methods. 1845 



seen to come to a focus squarely in front of the center of the condenser. Apiece 

 of paper held for a moment in the focus proves the necessity of the water tank, 

 or heat screen (w), which should be filled with perfectly clear water. Rotate the 

 mirror and adjust its slant so as to keep the focus of the light in the optical axis 

 of the condenser and tank, and set the microscope in its place. Too much 

 emphasis can not be laid on the rule that objectives of different powers must be 

 set at different distances from the condenser. This distance is most readily measured 

 from that face of the condenser nearer the stage to the plane of the surface of the 

 stage. The following measurements were made on a porte-lumiere fitted with a 

 single condenser of four and a half inches diameter and six inches focal length 

 combined with a sub-stage condenser having a focal length of thirteen-sixteenths 

 of an inch. They may be used as a guide in determining the proper adjustment 

 of a microscope to condensers of different focal lengths by applying the rule that 

 the distances are increased when a condenser of longer focal length is used, and 

 decreased when using condensers of shorter focal length. The distance from 

 the surface of the condenser nearer to the water tank (w) to the surface of the 

 stage was as follows, with 1 in. Wales objective 8y\ inches, No. 3 (or ^ in.) 

 Leitz lyi inches. No. 6 (or i in.), Leitz 6^ inches, and B. & L. ^Vinch 

 oil-immersion 5^% inches. 



To find the position giving maximum and evenly distributed illumination over 

 the entire field on the screen with any given combination of condensers and 

 objective, proceed as follows : (1) Adjust the mirror so that the light will be 

 reflected along the optical axis of the condenser and microscope. (2) Place a 

 mounted microscopical object on the stage and bring a low power objective to 

 an approximate focus upon it. (3) If the light on the screen is at first too faint 

 to permit of accurate focusing, move the microscope nearer to the water tank 

 and focus sharply. (4) The object may now be removed from the stage, provided 

 the focus of the objective remains unchanged. Mark the position of the micro- 

 scope and test the light when the microscope is moved nearer to the water tank 

 or farther from it. If the field of illumination on the screen shows a blue color, 

 the microscope is too near the water tank. If the light is faint, even at its best, 

 the mirror may need adjusting, the condenser may not be set squarely in its cell 

 so that the condenser and microscope are not in the same optical axis, or the 

 focal length of the condenser may not be suited to the objective. When the best 

 distance has been determined for each objective, make a permanent record for 

 future reference. (5) A strong light, either pale blue, white, or slightly yel- 

 low, may be obtained by varying the distance of the microscope from the condenser. 

 The yellow tint is to be chosen as it gives the best differentation of transparent 

 tissues. (6) A sheet of foolscap or other white paper should always be at hand 

 ready to be used as a temporary screen between the condenser and water tank. 

 One use for it is to cut off the heat from the objective whenever work is inter- 

 rupted for a time, but its most important use is to so reduce the intensity of the 

 light which is reflected laterally from the polished end of the objective that 

 vision will not be blurred when looking at the objective in focusing down on an 

 object. Opaque or semi-opaque objects mounted in balsam should be protected 

 from the heat as much as possible, for they are much more likely to be damaged 



