FUTUllE OF THE MICKOSCOPE 39 



an oil immersion objective, for instance, a common method is to 

 immerse tke objective and then lower it so that it all but touches 

 the top surface of the cover glass. The objective is then raised 

 by means of the fine adjustment until the object comes into view. 

 While this may act fairly well with very thin cover glasses, it is 

 a haphazard method when cover glasses of varying thicknesses are 

 used. It should be realised that when microscope users are sufii- 

 ciently educated, they will be able to tell how far they are from 

 the actual image by the appearance of the light in the field of view, 

 that is, if the object is illuminated with reasonable accuracy. 



Mechanical stages also appear to me to need some consideration. 

 The stages which will on actuation cause no shift of the object 

 other than in the direction intended, or any alteration of focus, 

 are rare. Further, those in which the screws project over the side 

 for a considerable distance with the result that any slight jar or 

 knock causes them to be displaced, and, it may be, actually bent, 

 are objectionable when used under laboratory conditions. 



There is, I think, much to be said for the type of stage which 

 has either co-axial milled heads on a vertical axis, or, if incon- 

 venient to make, milled heads which are on separate axes. This 

 method of construction, I think, of necessity results in a much 

 stiffer and more stable stage. There is, in fact, a general lack of 

 stability going through nearly all parts of a microscope. But it 

 is significant that, even so long ago as the beginning of last century, 

 the instrument as then designed had much greater attention paid 

 to this point. The microscope, an illustration of which I show on 

 the screen, is to my mind an embodiment of a principle that should 

 receive attention. So soon as English makers are in a position to 

 consider the production of an instrument of a special type, it is 

 my intention to have one made. In this the general principle is 

 that all the optical parts are carried on a bar which is, in effect, 

 an optical bench, and that this is strutted in such a way as to give 

 stiffness to the instrument as a whole. The only effort that I am 

 aware of that has been made in this direction is in the microscope 

 designed by Dr. Rosenhain, partichiarly for metallography, but 

 which is adaptable for ordinary work. This instrument, to my 

 mind, is such an improvement on any other type of stand that I 

 am at a loss to understand why metallographers have not more 

 generally taken it up. It might appear that I am exaggerating 

 the importance of stability in the stand. But it should be realised 

 that any want of centration in the optical parts, or want of align- 

 ment in the optic axes of these parts, results in more serious dete- 

 rioration of the resulting microscopic image than any other single 

 factor. The optical parts of a microscope are the objective, 

 for obtaining the primary magnified image of the object : the 

 ocular, for further enlarging that image and transmitting it to the 

 eye ; and the sub-stage condenser, for illuminating the object with 

 a larger or smaller cone of light. The limitations of time will pre- 

 vent me from doing more than refer very briefly to some properties of 

 the optical parts. 



It is generally assumed that magnification is the primary func- 

 tion of an objective. But in point of fact the main point is not 

 magnification but resolution. By resolution is meant the power the 



