38 THE PRESENT POSITION AND THE 



A consideration of the microscope resolves itself of necessity 

 into two parts, the mechanical and the optical. From the mech- 

 anical standpoint there are two designs in general use — those referred 

 to as the Continental and the English form of microscope. In the 

 Continental type it has usually been customary to have what is 

 known as the horseshoe foot, mainly, I imagine, because of its ease 

 of construction by mechanical engineering methods; whereas the 

 English design of microscope, which has hitherto been mainly made 

 by hand, is of a more steady type, and the points of support are 

 so distributed as to give more stability to the instrument in any 

 position. 



The essential parts of the instruments are a coarse adjustment, 

 to give the body tube a quick motion in the direction of the optic 

 axis, and a fine adjustment, which gives it a much slower motion 

 in the same direction. The tube is adjustable in length, to enable 

 correction to be made for varying thicknesses of cover glass, althoiign 

 a large number of workers appear to regard it as a ready method 

 of obtaining greater or less magnification, with disastrous effects 

 on the resulting image. 



There is only one fixed part of a microscope which is used for 

 biological purposes, and that is the stage. But metallographers 

 require that the stage shall also be adjustable in the direction of 

 the optic axis. The body tube itself should be made so that it 

 can be closed to a length of 140 millimetres, including any objective 

 changing device that may be on the nose-piece ; and it should be 

 possible to lengthen it to at least 200 millimetres or 250 millimetres 

 if long-tube objectives are used. 



All these adjustments are in the direction of the optic axis of 

 the instrument. Two others are usually provided, which are at 

 right angles to this direction — that is, a mechanical stage for actuat- 

 ing the object, and in certain of the best class instruments an 

 arrangement for centering the sub-stage condenser to the axis of 

 the objective. While there are many points which might be raised 

 on the mechanical side, there are only one or two that I have time 

 to mention. The main points about most microscopes appears to 

 be that they are unstable. I have a considerable number in my 

 own possession, but I do not think I have one, even now, which, 

 if I centre an object on the stage with the instrument in a vertical 

 position, still maintains its centration accurately if the instrument 

 is put into the horizontal. The probability is, therefore, that there 

 are few microscopes made at the present time that exactly fulfil 

 the conditions necessary for high-class photomicrographic work, or 

 for observational microscopic work of an exacting order. I trust, 

 however, that an instrument exhibited at this Symposium will embody 

 LJie necessary improvements to rectify this matter. 



Some misa})])rehension a])])ears to me to exist also as to the rela- 

 tive purpose of the coarse and the fine adjustments. The coarse 

 adjustment should be sufficiently well made, and if the user is 

 sufficiently expert, to enable him to bring into view any object, 

 whether it is being observed with a low or a high power objective. 

 The fine adjustment is then used for accurate focussing and for get- 

 ting a conception of the object in de])th. In biological work, at any 

 rate, this is very rarely the state of affairs as carried out. In using 



