chromatic objectives are to an increasing extent supplanting the older objectives, 

 yet there are a great many problems in microscopy that do not demand the 

 highest attainable degree of optical perfection, and in the majority of such cases 

 the older "achromatic" microscope will render as good and sufficient service as 

 ever, provided the instrument be good of its kind, i. e., skilfully and carefully 

 made. The objectives and oculars of the older type have certainly this advant- 

 age that, thanks to their much simpler construction, really good lenses of this 

 class can be supplied at prices considerably lower than those of the lenses of the 

 new series, which are much more complicated and involve in their production 

 an extraordinary degree of manual skill. 



We should not omit to mention that all advantages derived 

 from the introduction of new glasses and of the fluorites, and the 

 adoption of improved formulae have been applied equally to the 

 objectives of the older type. The improvements thus effected are of con- 

 siderable importance, especially in the higher powers of the dry and immersion 

 series, which may therefore, after the example of other makers of similar con- 

 structions, appropriately be termed "Semi-apochromatic objectives", 

 or to part, also "Fluorite Systems". 



To the two types of objectives just mentioned there has been added recently 

 yet a third, computed by VON RoHR: the type of the Monochromats. The 

 special features of this system are : a very perfect union of rays (spherical correction) 

 for light rays of a definite wave-length chosen at will, the lack of a correction of 

 chromatic aberration and the composition of the system from uncemented single 

 lenses, which are all formed out of the same material. Since there is no chromatic 

 correction at all, this system can only be employed in illumination with strictly 

 monochromatic light. 



Such objectives are manufactured for the present only for the Micro-photo- 

 graphic Appliances for Ultra-violet Light, spoken of in the preface, since the mono- 

 chromatic light employed makes objectives of this correction appear suitable. 

 They are corrected for a wave-length 275 (iji, all lenses consisting of molten 

 quartz. We also supply a series of oculars, specially for use with these Mono- 

 chromats for ultra-violet light whose lenses are made from quartz crystals. 



Since 1808 all our manufacturing processes follow the system introduced 

 into our works by Professor ABBE, a system which is based upon complete 

 theoretical computation of all the constructive data previous to 

 actual manufacture. By this method it has become possible, through judicious division 

 of labour in the various stages of manufacture under competent scientific super- 

 vision, to employ a large number of operatives in the production of lenses of 

 the greatest possible perfection. 



The necessity of empirical tests has been entirely obviated by precise mathe- 

 matical computation of every detail of construction combined with exact technical 

 methods and systematic control of each phase of manufacture. This ensures an 



