with relatively great focal lengths and yielding, with the one objective, an exten- 

 sive series of magnifications. 



The list of apochromatic objectives on page 15 specifies in addition to par- 

 ticulars of the numerical aperture and foci also the corresponding initial magnifi- 

 cation, i. e., the magnification which the objective alone would give at the distance 

 of distinct vision if used as a single lens. This is simply 25U (distance of distinct 

 vision in millimetres) divided by the focal length of the objective (also given in 

 millimetres). For instance, the initial magnification of a 3 mm objective is: 



250 



The numerical apertures indicated in the table are the guar- 

 anteed minimum values. 



Durability of the new Glasses. 



In reply to numerous enquiries and, in some cases, publicly expressed 

 doubts we would here state that the kinds of glass used by us since 1 *;>,"> in 

 the construction of apochromatic lenses, and in a limited degree for the higher 

 powers of achromatic objectives also, have now been so amply tested as to offer 

 an absolute guarantee for their permanency. 



In all cases where one of the lenses of an objective supplied by us 

 should exhibit spontaneous changes, we shall continue, in the future (as 

 we have done in the past), to repair such objectives at our own cost, 

 substituting a new lens made of glass which has been thoroughly tested 

 and proved to be permanent. 



To this promise we attach, however, the condition that the objective in 

 question be sent to us without previous meddling by others - - such as dis- 

 mounting with a view to cleaning - - since any interference, by inexperienced 

 hands, tends in many cases to convert a slight irregularity, easily rectified by 

 us, into a serious, or even irreparable, defect. 



We should also point out that owing to the hyper-hemispherical form of 

 the front lens, which is held in position by a very narrow ridge at the extreme 

 edge of the setting, the apochromatic homogeneous immersion objec- 

 tives of 2 mm and 3 mm focus and 1.40 num. A p. demand very careful 

 treatment. Even a slight jar by contact with the object may loosen this lens in 

 its mount. 



