53o 



Fig. 7. 



narrower than 13 m.m., as there are technical objections against 

 the construciion of apocliromatical objectives of high aperture with 

 an external diameter less than 12 ni.m. Another Avay consists in 

 increasing the field-current as much as possible. We have then a double 

 advantage. ' First!}- the maximal field-intensity is increased as much 

 as possible. Secondly with the more perfect magnetic saturation of 

 the iron core, the magnetic leakage near the central bore pushes 

 a relatively greater number of lines of force into the part of the 

 field corresponding to the bore. The central part of the field increases 

 thereby both absolutely and relatively. The direct measurement 

 immediately confirms this. 



Using the bismuth-method I found with the electro-magnet of 

 galvanometer II in the weakest part a field with an average intensity 

 of 14J50 gauss, when the strongest part reached 31350 gauss. With 

 a maximal field-strength of 20950 gauss the field in tiie central part 

 fell to 5650 gauss. 



With a new electro-magnet of a somewhat different form and size, 

 which is wound for 25 ampère, I found : 



at 21 ampère maximal 39050 minimal : 23200 

 „ 10 „ „ 36250 „ 19760 



„ 1 „ „ 17010 „ 7417 



