152 



Note on the Determination of "Optical Tube Length." 

 By a. Ashe. 



{Read October 21st, 1892.) 



This is one of those practical matters the investigation of 

 which many microscopists postpone indefinitely, and generally 

 end by neglecting entirely, under the mistaken impression that 

 its solution is involved in much difficulty, requiring an advanced 

 knowledge of the laws of optics and a large amount of manipu- 

 lative dexterity in order to arrive at a satisfactory result, and 

 that even if a correct measurement can be made the informa- 

 tion so obtained is of no real value to the worker. 



The fallacy, however, of this latter view is so obvious that it 

 needs no refutation to anyone who has taken the trouble to 

 estimate the magnifying power of his own instrument. 



To those who are content to accept the figures given in an 

 optician's list as to the amplification of their various lenses the 

 following quotation from Mr. Crisp's well-known article may 

 carry some weight : — 



" Microscopists have always recognized that the length of the 

 tube of a microscope is a factor in determining the amplifica- 

 tion of the image, that the amplification is generally greater 

 with a 10-inch tube than with one of six inches, and that we 

 obtain an increase of power by pulling out the draw-tube. 

 Here, however, all exact notions as to the functions of the 

 tube length have practically stopped, so much so that there has 

 not been any agreement even as to how the length of the tube 

 is to be measured, whether from the front or back lens of the 

 objective to the field-lens, the diaphragm, or the eye-lens of the 

 eye-piece." 



Since these lines were written, now some eight years ago, it 

 has come to be very generally admitted that the optical tube 

 length must be measured from the posterior principal focal plane 

 of the objective to the anterior principal focal plane of the ocular. 



