THE president's ADDRESS. 359 



the troughs, except the first one, next to the eye-piece of 

 the microscope, which is secui^ely attached to one of the 

 troughs. We will now describe the tubes ; the first has 

 a metal front, with a brass tube large enough for the body of 

 the microscope to enter freely ; this tube has a door in the left- 

 hand side, also a very light vulcanite drop shutter, which can 

 be released by a knob on the outside. 



The intermediate tubes each contain nothing but a black- 

 ened diaphragm, for the purpose of cutting out internal re- 

 flections ; there are four intermediate tubes, viz., two of one foot, 

 one of two feet, and one of six inches. The joints are made so 

 that they are interchangeable and light tight, even though they 

 should not be placed very firmly together. These tubes are 

 quite unimportant, and might be made of mill board. There 

 is an end tube which contains a vulcanite flap shutter worked 

 by a milled head on the outside ; the back of this tube is 

 grooved, so as to take the dark slide, ground glass, and repeat- 

 ing back. Close to the back is a fitting to hold metallic masks 

 of various sizes. The ground glass is an important part which 

 requires special attention. It is rather anomalous to call it a 

 ground glass, as the final focussing is performed on clear glass; 

 nevertheless a ground glass is indispensable for the purpose of 

 arranging and roughly focussing the object upon. 



The arrangement before you, which was designed in 1886, has 

 been in constant use, and subsequent experience has suggested 

 no improvement. The plane glass is held in a wooden frame, 

 which is grooved so that it may slide in the back ; on the 

 plane glass, across the top and bottom, is ruled a scale of 

 inches and mm. ; the centre of the plate is denoted by faint 

 diagonals ; to the top of this frame is hinged the ground glass, 

 and it is so arranged that it will remain up, when placed in that 

 position. This is a more convenient plan than that of having 

 two separate slides, one for the plane, and one for the ground 

 glass. Further, the scales on the plane glass form an excellent 

 micrometer for determining the magnifying power, as well as 

 the size of objects ; as these scales are ruled at the top and 

 bottom of the glass, they in no wise interfere with the vision 

 about the centre ; the bottom scale can be brought to the 

 centre, for micrometric and other purposes, by raising the 

 frame, and the top one by its inversion. The back takes 



