234 Royal Society : — 



against the other. The divisions on the outer limb have their zero 

 on the left, and the 100 division on the right; on the inside limb the 

 divisions are in a contrary direction. 



By means of the endless screw X acting on the toothed edge 

 of the sextant, it can be moved on its horizontal axis, so that any of 

 its divisions mav be brought under the index fixed on the middle bar 

 QQ'. 



Now, supposing that by the table of Dr. Sommer the lens A 

 for a certain distance of the object should move 0*235, and the lens 

 B 0*765 of the whole space by which the lenses require to be sepa- 

 rated or approximated, we turn the endless screw until the index is 

 on the 23f division of the inside scale, and of course on the 76^ divi- 

 sion of the outside scale. 



In that position of the sextant the slits M M and M' M', by means 

 of the pins attached to the tubes of the lens A and of the lens B, will 

 make them accordingly move — A in the proportion of 0*235, and B 

 in the proportion of 0*765 of the whole space. 



If for another distance the lens A should have to move 0*333, 

 and the lens B 0*G66, setting both limbs of the sextant to these divi- 

 sions, the lens A will move j, and the lens B § of the whole space. 



If we wanted to move the two lenses in the same proportion, the 

 sextant should be set so that the 50th division of both scales should 

 be under the index. 



And, finally, if, for the sake of comparative experiments, it were 

 wanted to move only the lens A or the lens B, the slit of the 

 pin for either and the zero of the scale should be placed under the 

 index, by which that lens would be completely stationary, and the 

 whole motion imparted to the other. 



Latest Improvements, since the Communication to the Royal Society. 



In the construction of the apparatus it was necessary to reduce 

 to its minimum the friction of the various parts of the instrument and 

 to render the action as smooth and easy as possible ; otherwise the 

 force necessary to overcome resistance would almost inevitably cause 

 vibration of the whole apparatus, and thereby of the optical image 

 on the sensitive surface, and consequently confusion in the ultimate 

 formation of the photographic picture. 



The lenses being fixed, each on its separate tube, one sliding 

 in the other, in order to prevent the vibration of the two lenses during 

 the movement, the tubes must fit very closely, in which case the 

 friction is very great, and the evil described above occurs inevitably. 

 To avoid this, the two tubes holding the lenses are made sufficiently 

 different in size from the external tube, so as to leave a free space be- 

 tween them. Now a steady and regular movement is obtained by 

 means of a slit made lengthwise in the top of the external tube, 

 through which the pins attached to the two moving tubes holding 

 the lenses can slide very freely. By means of a flat head placed at 

 the top of each pin, and supporting the tube holding the lens, the 

 whole friction is reduced to the bearing of the flat heads of the pins 

 whiie they are sliding on the narrow strips forming the edges of the 



