OhservaUo7is on Amphipleura pellueida. By J. J. Woodward. 665 



For this purpose I had selected an " amplifier " made for the 

 Museum more than ten years ago hy Mr. Tolles, of Boston. It 

 was originally intended to be used at the end of the draw-tube in 

 ordinary microscopic work for the purpose of obtaining increased 

 magnifying power. This " amplifier " was a negative achromatic 

 meniscus of about 6 • 5 inches virtual focus and • 7 of an inch in 

 diameter. I had in the Museum collection a number of other 

 " amplifiers " specially constructed for the projection of microscopic 

 images, but knew by previous experience that none of them 

 equalled the Tolles amplifier in flatness of field or freedom from 

 spherical aberration. After failure with the Zeiss amplifier, there- 

 fore, I tried this Tolles amplifier in the manner I had originally 

 intended, and after some little experiment to ascertain its proper 

 position in the Microscope body for the distances at which I de- 

 signed to form the images, I obtained the results at which I aimed. 



It will readily be understood that the best position of the 

 amplifier with the Zeiss I or -J^ for the projection of the image to 

 say 10 feet distance from the object, will also be the best position 

 for all other objectives, provided their corrections are best neutralized 

 when they are focussed on the object with the 10-inch tube, and 

 therefore the positions of the draw-tube corresponding to the best 

 images at a series of selected distances having been found by careful 

 experiment with any selected objective, may be used successfully 

 with any other objective corrected for the 10-inch tube. Indeed 

 this method may be advantageously resorted to in the case of 

 objectives provided with a screw collar, in place of the usual method 

 in which the screw collar is used to correct the aberrations intro- 

 duced by distance. The positions of the amplifier, as indicated by 

 the scale on the draw-tube, having been found for the distances at 

 which the pictures are to be taken, it is only necessary in sub- 

 sequent work with the same amplifier, to set it, by means of the 

 draw-tube, at the position known to be best for the distance 

 selected, and then to focus with the fine adjustment in the usual 

 way. 



It was in this manner that the images were projected for the 

 photographs I sent to Zeiss, and for those which accompany this 

 paper. I am pleased to observe by his printed circular, dated 

 March, 1879, that since he received my letter in which this method 

 was explained, and the photographs sent with it, Zeiss has been 

 influenced by them to construct and advertise for sale a concave 

 lens of 10 or 12 centimetres focus, to be placed at the end of the 

 draw-tube (in das Auszugsrohr des Tubus eingesetzt), for the 

 purpose of projecting images with his objectives, instead of the 

 unsatisfactory device he ofiered me last fall. I regret to find, 

 however, that I have unintentionally misled Zeiss as to the focal 

 length of the amplifier I used. Copying by accident from my 



