Price $8.—

Huyghenian Eye-pieces.

Number	0	I	II	III	IV	V
Focal length mm	50	40	35	30	25	20

Price of each eyepiece $ 2.00.

The objectives designed for the Edinger apparatus of 64, 42, 35, and 24 mm focal distance have been found to be very useful for a number of purposes and have come into great favor.

(see. Dr. Kaiserling: Praktikum der wissenschaftlichen Photographie p. 263).

Achromatic Objectives.

No. of Objective	Focal length	Numerical Aperture (num. aper.)	Micrometer Values	Price $
{ 1* { 1 Low power { 1a Dry Series { 2 { 3 { 4	44 mm (1-3/4")	0,09	0,054 mm = 54µ	3.25[1]
	44 mm (1-3/4")	0,09	0,054 mm = 54µ	6
	39-27 mm	0,06—0,10	50—26µ	10 (adjustable mounting)
	30 mm (1-1/4")	0,14	0,028 mm = 28µ	6
	18 mm (3/4)	0,28	0,015 mm = 16µ	6
	8 mm (1/3)	0,55	0,009 mm = 9µ	10
High power { 5 { 6 (Fluorite) { 6a { 7 (Fluorite) { 7a " { 8 " { 9	5,8 mm (1/4")	0,77	0,0045 mm = 4,5µ	10
	4,4 mm (1/6")	0,82	0,0034 mm = 3,5µ	12
	4,4 mm (1/6")	0,82	0,0034 mm = 3,5µ	16 [2]
	3,2 mm (1/8")	0,85	0,0026 mm = 2,6µ	12
	3,2 mm (1/8")	0,85	0,0026 mm = 2,6µ	16[2]
	2,5 mm (1/10")	0,87	0,0020 mm = 2,0µ	16
	2,2 mm (1/12")	0,87	0,0017 mm = 1,7µ	24
Water-Immersion 10	2,2 mm (1/12")	1,10	0,0017 mm = 1,7µ	26
{ 1/10 Homogeneous Oil-Immersion { 1/12 { 1/16	2,5 mm (1/10")	1,30	0,0022 mm = 2,2µ	30
	2,1 mm (1/12")	1,30	0,0017 mm = 1,7µ	40
	1,7 mm (1/16")	1,30	0,0014 mm = 1,4µ	60

[1] Objective 1* consists of only one doublet, carefully corrected. It is sufficient for many purposes, though it does not possess the same brilliancy of field as the No. 1 objective.