200 LECTURES TO SCIENCE TEACHERS. 



inclining the body, but have to look straight down. There 

 is a disadvantage in that because the tear of the eye runs 

 over the pupil, and you do not get such good definition as 

 when you can incline the microscope to any convenient angle, 

 so that the tears may not remain in front of the pupil. 

 Some have the stage as simple as it is possible to be, the 

 body is a simple tube, and you get a coarse adjustment by 

 pulling it up and down. There is a reflector below the 

 stage, but scarcely anything which is not absolutely requisite. 



Many of the microscopes in the Exhibition have various 

 excellent and valuable movements. It is very important to 

 have no vibrating motion in the tube, because if you are in a 

 place where there is the least tremor, and the tube is able to 

 move in the least degree from side to side, the object is seen 

 to vibrate in the most inconvenient manner. Some of the 

 early microscopes are for this reason totally unfit for difficult 

 observations. I think it a disadvantage to have the body 

 move in any direction but up and down, unless the instrument 

 is very well turned out of hand. If it is made to perfection the 

 power of turning on one side may be very good for particular 

 purposes. An excellent stand for holding the tube is the 

 so-called Jackson model. The body, the stage and the sub- 

 stage, are all firmly held by one solid piece of metal, and must 

 therefore vibrate together in such a manner as to be of little im- 

 portance. The body slides up and down on a long groove with 

 rackwork, and there can be no more secure means of prevent- 

 ing lateral movement, because it is held so firmly for such a 

 distance. 



Proceeding to the stage, we have some of the most simple 

 form, and others with very complex movements, so that you 

 can move the objects up and down or from side to side by the 

 milled heads, or can rotate the whole round quickly or slowly 

 with the screw, the object remaining all the while in the 

 centre of the field. I might say much more on these various 

 arrangements if time would permit, but cannot, I think, do 

 better than express my general conclusions in a simple form. 

 I would strongly insist on the desirability of having every 

 accessory movement well made or not made at all. Whatever 

 movements there are should do only just what is required, 

 and should admit of no other motion, and this can only be 

 accomplished by good and careful workmanship. If you do 

 not wish to be at the expense of good workmanship you 





