38o 



THE GUIDE TO NATURE. 



unsymmetrical the speed is i-y or f-7.7, 

 depending" upon the differences in the 

 focal lengths of the two parts. As op- 

 posed to this we have the speed of f-6.3 

 in all sizes of the Tessar Series lib, 

 and the marvelous speed of f-4.5 in all 

 sizes of the Series Ic. This question of 

 speed is of utmost importance to the 

 naturalist, who frequently desires to 

 make negatives of animals in their na- 

 tural habitat where the work must be 

 instantaneous and yet the light condi- 

 tions are not always the most favorable. 

 For such work the Tessar Ic is abso- 

 lutely unsurpassed. 



Cameras limit the use of lenses in 

 two directions. First as to their size 

 and second as to their focal lengths. 

 For instance, one may desire to use the 

 Tessar Ic, and yet the camera which 

 he employs may be so small that the 

 front board will not contain the objec- 

 tive. In such cases the Tessar, Series 

 lib, should be employed if the convertible 

 lens is not wanted. Again, the Conver- 

 tible Protar may be desired in cases 

 where the bellows draw of a camera is 

 so short as to render useless the single- 



elements. In this case a symmetrical 

 Protar or a Tessar would be best. 



Another consideration is that of cost 

 and is mentioned here only to show that 

 the Convertible Protar, while apparently 

 the most expensive, is in reality the 

 cheapest Anastigmat, possessing such 

 superb qualities, that has ever been pro- 

 duced. When using three single ele- 

 ments giving six focal lengths it is but 

 fair to divide the total cost by six, 

 resulting in an average of a little over 

 $17.00 each for Anastigmats of the high- 

 est optical correction. And if another 

 single lens be added the result is four 

 single and six doublet lenses at a cost 

 of about $15.00 each. 



It will thus be seen that but little can 

 be done to actually decide what lenses 

 the naturalist should select. He must 

 consider the qualities possessed bv each 

 objective and decide for himself upon 

 those which lie can omit. The Bausch 

 & Lomb Optical Co., of Rochester, N. 

 Y., will be pleased to render whatever 

 assistance they may be able in solving 

 the individual problems of those inter- 

 ested. 



PHOTOGRAPHIC STUDY OF PLUMOSE TAILS OF CLEMATIS. 



