A SIMPLE DESCRIPTION OF THE MICROSCOPE 21 



adapter has screwed studs with clamping screws, which form the 

 stops in both directions when the object glass is in the correct 

 position. These can be adjusted by means of a spanner supplied 

 for the purpose, so that each lens can be centred with an accuracy 

 that is never possible with a revolving 

 nosepiece, because ' the error of each 

 individual object glass cannot be com- 

 pensated with the latter. 



The construction of this apparatus 

 is so simple and rigid, having no slides 

 to wear loose, that it remains in ad- 

 justment permanently. 



The total extra length of the micro- 



scope body caused by its use is 10 mm., -pia. 9 No 3281 Fit- 



and the drawtube should be set at ting of Sloan Object 

 150 mm. to obtain the standard tube Glass Changer, 

 length. 



A box is supplied to carry object glasses with fittings screwed 

 on ready for use, held against dust-tight pads. 



The illumination of an object seen with a microscope is ofiiiumina- 

 almost as much importance as the quality of the lenses. It is*^°°* 

 interesting to find that the methods worked out by those who 

 were enthusiastic in the use of the microscope as an enjoyment, 

 and to a great extent as an amusement, have been one by one 

 adopted by the more serious scientific worker who has sometimes 

 been ready to consider the time spent on the pure manipulation 

 of the instrument to be of little value. The proper use of the 

 substage condenser to regulate the light in viewing transparent 

 objects is now acknowledged to be of first importance for correct 

 observation. Dark-ground illumination, which has been considered 

 by some to be only useful to show in an attractive manner what 

 could be seen equally well by direct light, has proved to be of 

 paramount importance for the study of living bacteria and colloid 

 particles. The methods devised for illuminating opaque objects 

 have formed the basis for the observation of metallurgical 

 specimens, and the much- criticised study of the markings of 

 diatoms and insects' scales has proved to be of the greatest value 

 in enabling the images seen by the microscope to be correctly 

 interpreted. A bad lens can never be made to give a perfect 

 image, but a good lens will only give the best image when the 

 illumination is satisfactory. Most objects seen with the naked 

 eye only require that a sufficiently powerful light should fall upon 

 them. They reflect back the light that they receive, or the 

 greater portion of it, in all directions. It is not of importance 

 where the light which illuminates them comes from, although 

 occasionally, when the light falls upon them from one side only, 

 such deep shadows may be formed that it is difficult to recogm'se. 

 the true appearance. 



