INTRODUCTION. 



one to another the object is retained approximately in 

 focus. 1 



In all these points, except perhaps accurate centring, the 

 stands of good English makers probably equal, price for 

 price, the foreign stands ; while, on the contrary, price for 

 price, the eye-pieces and objectives of continental makers 

 usually are far superior to those made by the English 

 opticians a superiority probably due solely to the more 

 trained skill and more patient accuracy of the workmen. 



All of the work in this book, with perhaps the exception 

 of Chapters XXI. and XXXII., can be performed with the 

 the aid of objectives up to J ; but the student who has gained 

 some experience will probably wish to add to his microscope 

 one or more stronger objectives, in order to increase the range 

 and delicacy of his work. Increased magnification can be 

 obtained by increased power either of eye-piece or of object- 

 glass. All the objectives we have heretofore noted are what 

 are called " dry " systems, since they are used for work in a 

 dry state, and a layer of air separates the objective from the 

 object. " Dry " objectives of high power are subject to great 

 disadvantage from the serious loss of light their use involves. 

 The light, in passing from the mirror to the objective, passes 

 in the first place through air, then the object-slide, next the 

 object and the medium in which the object is mounted, which 

 may be glycerine, water, alcohol, etc., or even air ; then 

 through the cover-glass, and finally through air again. In 

 every one of these changes light is lost. Owing to this loss 

 of light, as well as for other reasons, it is not wise to use 

 high power eye-pieces with dry objectives; besides which, as 

 the eye-piece does not magnify the object, but only the 

 image of it as produced by the objective, any errors of this 

 latter are likewise magnified by the eye-piece. 



To obviate in part this loss of light, what are called " irn-. 



1 It is possible, by means of thin india-rubber or paper rings slipped on to 

 the shoulder of the objectives, to adjust their focal lengths with great accuracy. 

 In case of moderate inaccuracy of centring the student is advised to study 

 for himself the approximate position into which an object should be placed 

 when using the low power in order to bring it into the approximate centre of 

 the field when the higher power is turned into use. 



