toward the first end 136 a light emitter 110 is visible behind an optional transparent window 108 fig3 shows a view toward the end cap 106 at the second end 138 of the flashlight 102 in the example of fig1 the end cap battery adapter and flashlight body couple to one another by the engagement of threaded contact surfaces the contact surfaces may alternatively be adapted for a close sliding fit or a snap fit fig4 shows an example embodiment 100 arranged for operation with a primary battery when the primary battery is being used to power the flashlight the end cap 106 is attached directly to the body 104 and the battery adapter is omitted fig5 shows a side view of an example battery adapter 112 having an outer housing 113 in the example embodiment of fig5 a first end of the battery adapter is formed with an external thread 115 selected to engage with a corresponding internal thread on the flashlight body and with the internal thread 116 on another battery adapter 112 an internal thread 116 is formed near the second end opposite the externally threaded end an aperture passes through the battery adapter from the first end to the second end in some embodiments an optional hollow sleeve 114 has an inner diameter 124 that is slightly larger than the external diameter of a substitute battery giving a sliding fit of the substitute battery the hollow sleeve 114 may have an outer diameter 123 matching an inner diameter 150 of the battery compartment in the.