MONOCHROMATIC LIGHT AND EXPOSURE SHUTTER 107 



spectrum — the better the resolution by the objective ; but it is nevertheJess quite 

 true. Hence, microscopists have often desired some form of violet light to increase 

 the resolution of difficult objects. But the difficulty is this : if the fluid be 

 sufficiently dense to give pure violet light, the specimen is so feebly illuminated 

 as to be hardly visible, not enough, indeed, to enable the operator to form with 

 sufficient accuracy to produce a good picture even with a prolonged exposure. 

 Electric arc-light answers better than lime-light, as it contains more violet ray, 

 but sunlight is best. Both of these, it has been explained, are difficult to use 

 (see p. ii); notwithstanding this such specimens as amphipleura pellucida, when it 

 is required to show the lines really well as dots, demand its use, and with its 

 aid Dr. Van Heurck has produced photographs which are the admiration of all 

 photo-micrographers. We have found green light exceeding useful. 



(v) When using the mixed jet the heat from the lime-light is so intense that the 

 microscope becomes too hot, and the safety of the substage condenser, and most likely 

 that of the slide are imperilled. To obviate this, we often place a shield of brass 

 between the illuminant and the microscope, leaving but a small hole in its centre of 

 sufficient size to transmit enough light to fill the substage condenser. Its position is 

 shown in Figs. 24a, or c, page 54. This is only sufficient to prevent the heat striking 

 the microscope stand itself, it is true, but, nevertheless, is of a great value if the expo- 

 sure be a short one. Should, however, the operator have a very precious slide, or fear 

 his exposure may have to be several times repeated, it is better to use a w^ater-bath 

 (Fig. 24b, page 54). It should be large, and filled with glycerine and cold boiled water. 

 Some authorities recommend the addition of alum, but we have found no advantage 

 by so doing. Should the reader be one who is more or less of a beginner, we should 

 recommend him the use of the bath always^ whether the auxiliary condenser be used 

 or not. It is placed in our apparatus on the stand which holds the screen — shown 

 Fig. 24b, page 54. 



(vi) The Exposure Shutter. — There are many arrangements for this purpose. 

 Some prefer a simple black card placed in any suitable position to intercept the light, 

 others prefer some form, like a Thornton-Pickard, which is placed (actuated from 

 without by a pneumatic release) inside the camera. Ourselves, we use a strip of 

 zinc on a foot. It can be raised or lowered at will, and can be entirely removed from 

 the table when necessary. It is shown in Figs. 24a, and b, page 54, where it is tem- 

 porarily fixed half-way up, to make it show better. It is merely a movable horizontal 

 arm, working on a pillar supported by an iron foot. With high powers it is placed 

 betw^een the substage condenser and the coloured glass or water-trough ; but with 



