TUBE LENGTH AND COVER CLASS 



All objectives are corrected to a certain tube length 

 (i6omm by most makers Leitz i/omm) and all objec- 

 tives in fixed mounts of over .70 N. A. are corrected to a 

 definite thickness of cover glass as well. (Zeiss .I5mm 

 .2Omm; Leitz, .I7mm; Bausch & Lomb and Spencer, 

 .i8mm). These objectives give their best results only 

 when used with the cover glass and tube length for which 

 they are corrected. As indicated in the frontispiece, 

 the tube length extends from the eye lens of the eyepiece 

 to the end of the tube into which the objective or nose 

 piece is screwed. If a nosepiece is used the draw tube 

 must be correspondingly shortened. If the cover glass 

 is thinner than that for which the objective is corrected 

 the tube must be lengthened to obtain best results; 

 T thicker, shortened. The more expensive objectives 

 are provided with adjustable mounts by which the dis- 

 tances between the lens systems may be changed to com- 

 pensate for difference of thickness of cover. They are 

 successfully used only in the hands of an expert. One 

 of them out of adjustment is worse than an ordinary 

 objective. 



EQUIPMENT 



A microscope equipment may be very elaborate and 

 some work requires such an outfit, but every microscope 

 to be efficient should be provided with at least two 

 objectives, and preferably two oculars. If but one ocular 

 is bought the 6x (No. 2, or i J") is best. If two are pro- 

 vided, the 4x and 8x (No. I and 3, or 2" and i") are pref- 

 erable. For ordinary biological, histological and patho- 

 logical work the most desirable and most universally 

 used objectives are the i6mm (") and 4mm ("). The 

 equivalents of these in European nomenclature are Nos. 

 3 and 6, or A and D, according to the maker. In some 

 cases where a higher power is desired a 3mm (-J-" or No. 7) 



