APPENDIX 431 



automatic lamp where the arc has to be ' struck ' or formed at 

 starting by touching the carbons together by a hand movement, 

 and then separating them, the lamp then remains burning, the 

 carbons as they consume away being pushed up by springs against 

 steel screws, somewhat after the manner of the ordinary carriage 

 candle lamps, and this lamp also performs well where the current 

 is steady and not too high in voltage ; but in practice I myself prefer 

 hand-fed lamps to any on account of their simplicity and freedom 

 from any liability to fail or become unsteady at a critical moment. 



Of large automatic arc lamps little need be said ; those described 

 in Chapter XII. remain as the general pattern still in use. Messrs. 

 Oliver have brought one out on the same lines as the Brockie Pell 

 therein described, which is extremely steady and reliable, and is 

 in my opinion the best of such lamps at present on the market. 



Messrs. Newton also make large hand-fed lamps for their 

 triple rotating and similar lanterns, and these are by some operators 

 preferred to even the ' Oliver ' lamp. 



Projection Microscope. Various slight improvements, both 

 optical and mechanical, have been made in this instrument since 

 1890. Of these the most important has been the introduction of a 

 quadruple condenser, with a view to still further reducing spherical 

 aberration in the lantern portion of the instrument. After many 

 years of experiment, the new condenser, as designed by my father, 

 reduces this fault to a minimum, and a considerable gain in both 

 light and definition is the result. With the new arrangement 

 apochromatic lenses especially show to more advantage than formerly . 

 Of lenses, several new ones have been brought out which perform 

 well with the projection microscope. Herr Beichert has intro- 

 duced an oil-immersion lens, nominally but really ^ focus, of 

 N A 1-30 to 1'32, which for moderately high power is as good an 

 all-round projection-lens as I know of. As this lens has a very great 

 working distance, it is as well to ask for a shortening of this if 

 required chiefly for projection, as the horizontal position tends to 

 drain the oil away. 



The Zeiss apochromatic J of N A T40 does magnificent work, 

 but is very costly. 



I can also speak from experience of. Messrs. Swift's ^ oil 

 immersion at 51. 5s. ; this also for all-round work can hardly be 

 beaten, and Messrs. Leitz also do a fine series at a moderate price. 



Of mechanical improvements the most important has been the 



