MIRROR, STAGE, NOSEl'IECE, AND DRAWTUBE 143 



objective, when the microscope is again focused, comes 

 farther from the cover, and the magnification (and splierical 

 correction) is diminished; and vice versa. By adding an 

 amphfier, which is a corrected concave doublet, behind an 

 objective, the rays would be brought to a focus farther from 

 the object, and if the objective is raised enough, can be 

 focused at the standard tube length, causing an increase 

 of the working distance and a correction for a thick cover. 

 The homal is an amplifier used for projection at a distance, 

 and is so made as to also flatten the field; giving a magnifi- 

 cation, on the screen, in the Phoku camera, of 5 or 4.7. 

 (A homal of lower power is now made for low objectives.) 

 For low and high apertures and foci, the amount of flatten- 

 ing required is different. These homals are used by 

 Siedentopf in the new photographic attachment. A 

 concave achromatic amplifier is also used in the binocular 

 attachment of Siedentopf (and others) ; and should be so 

 arranged that the object, when centered on the monocular, 

 would still be centered when the eyepiece is removed 

 and replaced by the binocular attachment. Such an 

 amplifier was used by Van Heurck with high powers 

 without injury to the images, presumably the markings of 

 diatoms. 



Nelson (46) mentions the importance of a diaphragm 

 in the drawtube, 14 millimeters in diameter, and about a 

 centimeter below the longest eyepiece. This improves 

 the image by cutting off light reflected from the inside of 

 the tube. If the source of light is diaphragmed so that its 

 image is equal to the size of the field of view, however, 

 there is little extraneous light to be reflected from the sides 

 of the tube into the eyepiece. Nevertheless, such a 

 diaphragm might well be inserted, since it costs little 

 trouble, and is useful with a large source of light; and many 

 little things, each of which improves the vision almost 

 imperceptibly, add up and make a perceptible total. 



A rackwork on the drawtube would be of advantage to 

 one who uses high dry objectives without correction 

 collars, or who regularly uses the 8-millimeter apochromatic 



