THE CAMERA. 



379 



like an immense sponge with hundreds 

 of pores, from which in tiny streams the 

 water escapes from its underground 

 prison. This mass is the distinguishing 

 feature of one of the numerous small 

 ravines in the vicinity of Rochester, N. 

 Y., and, though accessible without great 

 effort, being within a half mile of a trol- 

 ley station and but a few hundred yards 

 of a public highway, its visitors are few, 

 and its being is not generally known 

 even to those whose inclinations are in 

 the direction of the natural and scien- 

 tific. 



THE TESSA K AM) PROTAR. 



(Published to answer the inquiries as to 

 differences between the two lenses.) 



When called upon to choose between 

 those two estimable lenses — the Bausch 

 & Lomb Zeiss Tessar and the Bausch & 

 Lomb Zeiss Protar — the naturalist has 

 before him a problem which is rather dif- 

 ficult of solution. Indeed the solution de- 

 pends upon the kind of camera in use and 

 the work to be done, each case having to 

 be considered individually. 



Both series of lenses are constructed 

 from the famous Jena glass, and arc 

 made equally precise in all details. The 

 corrections for astigmatism, for chro- 

 matic and spherical aberrations are 

 equally perfect, and the faithfulness 

 with which every point is delineated 

 upon the negative is in all respects iden- 

 tical with the two series. Optical per- 

 fection, therefore, and mechanical work- 

 manship are as wonderful in one case 

 as the other. 



Let us first consider one of the princi- 

 nal differences, that of convertibility. 

 The Tessar, both Series Ic and lib, 

 is an unsymmetrica! lens composed of 

 four elements which are divided into 

 two groups by the diaphragm. The 

 front lenses are separated by an air 

 space which acts as a lens, while the 

 rear elements are cemented. Neither of 

 these two dissimilar groups is optically 

 corrected, but the two when used in 

 con] unction exactly balance each other's 

 faults, thus producing a perfect lens. 

 The Tessar. therefore, must be used in 

 its entirety. And the components of the 

 two Tessars of different focal lengths 

 cannot be combined to produce a doub- 



THE INTERESTING EROSION. 

 In ravine at Rochester, New York. 



let. ( )n the other hand, the Protar is 

 so constructed that each of the doublets 

 is composed of two elements, either 

 identical or not, which may be used as 

 individual lenses. This construction 

 gives in the case of the symmetrical lens 

 a doublet, the focal length of which is 

 much shorter than either of the com- 

 ponents, while the single lenses may be 

 used as such. When the elements are 

 dissimilar in their focal lengths, three 

 lenses are in reality at command ; the 

 single lenses being of different focal 

 length give different size pictures of an 

 object from a given point and when 

 combined produce a perfect doublet. 

 Further, another cell may be added to 

 the first two, resulting in three single 

 lenses and three doublets, or six different 

 focal lengths all told. 



But with this advantage there is a 

 disadvantage as regards speed — that is, 

 some of the doublets will require longer 

 exposures than others, while all of the 

 single lenses work at an aperture of 

 f-12.5. When the doublet is symmetri- 

 cal the speed attainable is f-6.3 ; when 



