13 



be adjusted from the interior of the microscope room, but this is 

 not essential. If a small mirror be attached to the screen, it will 

 indicate the position of the screen that will reflect to the micro- 

 scope mirror a maximum of white light. Place the screen so 

 that the flash of sunlight from the mirrof strikes in the vicinity 

 of the microscope. Then of course, the whole of the screen will 

 be in a corresponding position and will be reflecting a maximum 

 of light. It is found that if the screen be placed in this position 

 for several hours the light from it remains practically con- 

 stant, so that while an adjustment by cords from the interior is 

 a convenience, it is not a very great necessity. If an adjustable 

 screen is not available, it is generally best to arrange one or 

 two fixed screens and thus accomplish the same object, one 

 screen for morning and another for afternoon. The light 

 from' a blue sky is not a satisfactory light. A white cloud gives 

 a very good light, but clouds are such fickle things that it is not 

 wise to rely upon them where the microscope is in constant use. 

 It is much preferable to construct a screen that will be avail- 

 able in a fixed position whenever the sun shines. When the 

 sun does not shine the sky must serve. 



It remains to say a few words in regard to the dissecting 

 microscope. This, as before mentioned, rests on the left hand 

 sliding table. The main microscope window is flanked by two 

 narrower windows, (forming a "bay") each of which is fitted with 

 opaque wooden screens sliding up and down with sash weights. 

 Whenever necessary, these screens can be raised and the light 

 necessary for the preparation of objects secured. The object 

 ready for examination, the screen is dropped, the room is dark- 

 ened and the examination with the compound microscope pro- 

 ceeds. The screens just described have a second purpose in the 

 carriage of batteries of accessory microscopes. It is often con- 

 venient in making comparisons to have several microscopes in 

 operation at one time, even where, on account of the cost, it is 

 not feasible to furnish complete outfits. To meet this end, cylin- 

 drical barrels capable of carrying objectives and eye-pieces are 

 mounted in batteries on the screens and are pointed at the sky. 

 The outfit that is found most convenient for each unit of this 

 battery is as follows : a rather higli-power eye-piece, a barrel i6o 

 mm. long capable of carrying the ordinary objective, and two 

 objectives, one of which is a moderate power corresponding to a 

 one-sixth Bausch and Lomb or a Zeiss "D," and the other a 

 two-thirds objective or Zeiss "A." 'In the wooden stage, a small 

 section of tu'binig of the same size, and capable of carrying an 

 objective, is used to carry the lower power of the two objectives. 

 It is found that by pairing these objectives with care the low 



