266 now TO WORK 



distance with sunlight, and for objects covered by glass of that 

 particular substance. 



Dr. Maddox remarks, that when the edges of objects under the 

 higher powers present, on the grey glass screen, a faint tint of claret 

 on the one side, and of apple green on the other, that great sharp- 

 ness will often exist in the negative ; the errors of the pairs of lenses 

 balancing one another as regards the actinic focus. The roughness 

 of the screen will not. in all cases, permit of the eye determining 

 under sunlight the best focus for the minute markings, and some fine 

 diffusing surface must be chosen, as a well-washed sensitised collo- 

 dion plate either previous to drying, flowed by a solution of tannin 

 or albumen, or not, or the surface of plate glass covered by fine 

 weak starch paste recommended by Mr. Carey Lea, or the serum 

 of milk, as occasionally used by Dr. Maddox ; some, as Dr. Wood- 

 ward, employ plain glass, but if used without a coloured medium 

 intervening, there may be some risk to the eye in working with the 

 lower powers. The object can likewise be focussed through a 

 parallel polished plate of blue glass. Nothing hitherto has been 

 found more generally serviceable than the finely ground surface for 

 ordinary work. 



Should the object be situated nny distance from the thin 

 covering, i.e., have much of the mounting medium included within 

 that space, although the objective may visually appear to work 

 fairly through the depth, it is seldom that the negative of the 

 image proves satisfactory. It is far better to remount the object, 

 or select another. Indeed, for the finer work, it is advantageous 

 that the objects should lie closely on the under-surface of the 

 thin cover. Diatoms and such bodies may be dried on it and 

 photographed, or after drying they may be placed on the drop 

 of balsam warmed on the glass slide ; this may likewise be thin, 

 and certainly should not be thick. If there be any vibration from 

 unsteadiness of the apparatus, or from wind, the results will be 

 unsatisfactory. 



343. inserting the Plate. The suitable sized plate of properly 

 cleaned glass being selected, and the materials required set at hand in 

 the dark room or portion of the chamber darkened off for this purpose, 

 and lighted by a yellow light or a small oil lamp with yellow glass 

 shade, the plate is held by the sides between the fingers and thumb 

 of the left hand, face down, the back wiped carefully with a (fry wide 

 flat camel-hair wash tool, to remove small particles of cotton or dust ', 

 then taken hold by a pneumatic holder in the centre, and the face 

 dusted over with the brush. Before taking the holder in the left 

 hand, see that the neck and lip of the collodion bottle are perfectly 



