13G MEMORANDA. 



not liave a copy of the said work, though every one ought. 

 My opinion still remains the same. The Maltwood finder 

 works tolerably up to one eighth ; but, Avith a sixteenth 

 (which I chiefly use with the Diotomaceous tests), the figures 

 become so fearfully diluted and nebulous that they require a 

 finder, i. e., lower power, to find them ! 



I have recently discovered another useful application of 

 the nose-piece. It does admirably for comparing two achro- 

 matics of the same power, in ordei' to ascertain which is the 

 best. The ordinary tedious mode of screwing and unscrew- 

 ing is very objectionable, as so much time is lost that the 

 observer cannot satisfactorily bear in mind the two effects. 

 With the nose-piece the change is made in an instant ; both 

 are brought, as we may say, close together, and may thus be 

 very accurately estimated. 



In this way I have been carefully comparing two recent Ig 

 inch achromatics by two of our first makers ; and the result 

 is, that no perceptible difference can be detected : Avhicli* 

 shows, by the way, how wonderfully our opticians work up to 

 each other. On the other hand, if we thus compare an 

 achromatic of the present day with another of the same 

 power and maker, but constructed, perhaps, only a year or 

 two ago, it strikingly shows the rapid improvement made in 

 achromatics ; every slight alteration of curve, density of 

 glass, variation of combination, &c., having been productive 

 of more or less benefit. When shall we get to the top ? — 

 Henry U. Janson, Pennsylvania Park, Exeter. 



Binocular.— In answer to numerous private inquiries for ad- 

 vice, and a recommendation of the makers who will apply 

 my binocular adaptation to microscopes generally in the 

 best and most efficient manner, I have to state, that after a 

 careful examination of the instruments of the three who 

 have, up to this time, professed to construct them, I can pro- 

 nounce the definition of the binocular arrangement equally 

 good in all ; and as each is determined in making the new 

 instruments as perfect as possible, I feel assured that I can- 

 not do better than strongly recommend parties requiring 

 tlieir instruments to be altered to send them to the original 

 makers, who will certainly be best qualified for applying the 

 binocular arrangement to their OAvn particular instruments. 

 I am sure this will be most satisfactory in the end to micro- 

 scopists, as well as the opticians, and prevent the possibility 

 of any invidious comparisons being set afloat at the expense 

 of other instruments, for the purpose of obtaining business 

 by a self-assumed superiority of construction; :\ ^ourse of 



